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Annual Flowers and Plants

Annual flowers and foliage plants provide many options for short-term accent and beautification in a landscape. By definition, annuals are those plants that live and bloom for only one year. They die at the end of the growing season and must be replanted or reseed themselves the following season.

In some cases, desirable landscape plants such as impatiens, coleus and geraniums are actually tender perennials, meaning they live for many years in warm climates, but cannot survive our Idaho winters. We refer to and treat these plants as annuals in the garden.

The effort and expense of replacing plants each year are one drawback to using annuals in the garden. However, annuals are unmatched in variety, color, bloom period and adaptability. Many can be grown from seed for pennies a plant. There is an annual plant for every situation in the landscape and all these factors compensates for the extra effort involved in establishment, removal and replanting. Some annuals, such as cosmos, re-seed freely, establishing themselves in a garden year after year.

Annual flowers can be used in traditional beds, rock gardens, cutting gardens, borders, window boxes, containers and hanging baskets. They add interest and color to architectural features. They can also be used to beautify uninteresting areas in our landscapes or used as screens to hide undesirable features. Some annuals, like California poppy and sunflower, grow in dry, rocky, shallow areas where little else will grow and bloom.

Your local garden center will likely stock thousands of different annuals for the garden and containers, and exciting new cultivars are being released continually.

In this section you’ll find a complete guide to selecting, planting and caring for annual flowers and foliage plants.

Hardy Annuals

These plants tolerate frost and cool growing conditions.  Many bloom in early spring or can be planted in late fall to add color to the landscape.  Hardy annuals are irreplaceable for those who live in the cool, high-elevation areas of Idaho, but can be good additions to the spring and fall landscapes in the warmer valleys typical of southern Idaho.  Popular hardy annuals suitable for Idaho include:

Common Name Scientific Name Regional Adaptation
Alyssum Lobularia maritime N, SW, SC, SE, HA
Calendula Calendula officicinalis N, SW, SC, SE, HA
Cornflower Centaurea cyanus N, SW, SC, SE, HA
Dianthus (China Pink) Dianthus chinensis N, SW, SC, SE, HA
Dusty Miller Senecio cineraria N, SW, SC, SE, HA
Kale (ornamental cabbage) Brassica oleracea N, SW, SC, SE, HA
Larkspur Consolida ambigua N, SW, SC, SE, HA
Pansy Viola wittrockiana N, SW, SC, SE, HA
Snapdragon Antirrhinum majus N, SW, SC, SE, HA
Sweet Pea Lathyrus oderatus N, SW, SC, SE, HA
Viola Viola cornuta N, SW, SC, SE, HA

Semi-hardy Annuals

These plants are not injured by light frost events (down to 28° F).  They can also tolerate cool weather, although many are adapted to hot summer conditions and will bloom all summer.  Semi-hardy annuals can help extend the season and provide color into the fall.  Popular semi-hardy annuals suitable for Idaho include:

Common Name Scientific Name Regional Adaptation
Carnation Dianthus caryophyllus N, SW, SC, SE, HA
Cosmos Cosmos dipinnatus N, SW, SC, SE, HA
Gaillardia (Blanket Flower) Gaillardia pulchella N, SW, SC, SE, HA
Gazania Gazania rigeas N, SW, SC, SE, HA
Moss Rose Portulaca grandiflora SW, SC, SE
Petunia Petuniaspp. N, SW, SC, SE, HA
Phlox Phlox drummondii N, SW, SC, SE, HA
Rudbeckia Rudbeckia hirta N, SW, SC, SE, HA
Salvia Salviaspp. N, SW, SC, SE, HA
Sunflower Helianthus annuus N, SW, SC, SE, HA

Tender Annuals

These plants cannot withstand any frost and may not grow well during extended periods of cool weather.  Some of the tender annuals are actually perennials, but due to a lack of hardiness will only survive one summer in Idaho.  They can be planted only after the soil has warmed in the spring and will die back after the first frost of the fall, unless protected.  These plants are more suited to the warmer valleys of Idaho, although many can be planted in cooler areas if desired for a short season of summer color.  Popular tender annuals suitable for Idaho include:

Common Name Scientific Name Regional Adaptation
Ageratum Ageratum houstonianum N, SW, SC, SE
Amaranth (Love Lies Bleeding, Joseph’s Coat) Amaranthusspp. N, SW, SC, SE
Begonia Begonia x tuberhybrida N, SW, SC, SE
Celosia Celosia cristata N, SW, SC, SE
Chrysanthemum (annual) Chrysanthemum carinatum N, SW, SC, SE
Coleus Coleus x hybridus N, SW, SC, SE
Geranium Pelargonium hortorum N, SW, SC, SE, HA
Impatiens Impatiens wallerana N, SW, SC, SE
Marigold Tagetes patula N, SW, SC, SE, HA
Nicotiana (Flowering Tobacco) Nicotiana alata N, SW, SC, SE
Verbana Verbena x hybrida SW, SC
Zinnia Zinnia elegans N, SW, SC, SE

Notes

Key to regional adaptation notes:

  • N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
  • SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello. SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
  • SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
  • HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Purdue University publishes a frost hardiness chart for annual flowers.

Selection of the proper plant species and cultivars determine your ultimate success with annuals. Care must be taken to choose plants that complement their surroundings, are adapted to the local climate and situation, and provide the intended effect.

Annuals can be categorized based on growth characteristics or intended use, making selection easier. Useful groupings include hardiness, drought and heat tolerance, shade tolerance, fragrance, exhibition of ornamental foliage, ability to vine or climb, utility for hanging baskets, or use for cut flowers.

Additional outstanding information designed to provide assistance with selection of annual flowers is available from a number of sources. These include:

There are several internet sites with excellent pictures of annual flowers.

Some annual flowers can withstand hot, dry growing conditions and still provide attractive foliage and/or flowers. These plants may find their best use in the warm valleys of southwestern Idaho, though they will grow well in many other places in the state.

Common Name Scientific Name Regional Adaptation
Amaranth, (Love Lies Bleeding, Joseph’s Coat) Amaranthus spp. N, SW, SC, SE, HA
California Poppy Eschscholzia californica N, SW, SC, SE, HA
Celosia (Cockscomb) Celosia cristata N, SW, SC, SE
Dusty Miller Senecio cineraria N, SW, SC, SE, HA
Four-O’clock Mirabilis jalapa N, SW, SC, SE
Gaillardia (Annual Blanket Flower) Gaillardia pulchella N, SW, SC, SE, HA
Gazania Gazania rigeas N, SW, SC, SE, HA
Larkspur Delphinium ajacis N, SW, SC, SE, HA
Moss Rose Portulaca grandiflora SW, SC, SE
Phlox Phlox drummondii N, SW, SC, SE, HA
Rudbeckia Rudbeckia hirta N, SW, SC, SE, HA
Salvia Salvia spp. N, SW, SC, SE, HA
Sunflower (ornamental) Helianthus annuus N, SW, SC, SE, HA
Zinnia Zinnia elegans N, SW, SC, SE

Key to regional adaptation notes:

  • N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
  • SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello. SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
  • SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
  • HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Annual flowers differ in their ability to withstand shade while maintaining attractive foliage and producing blossoms. Most annuals require some full sunlight during the day, but here is a list of plants, suitable for Idaho, that can withstand part or full shade and still be attractive.

Common Name Scientific Name Regional Adaptation
Ageratum (P) Ageratum houstonianum N, SW, SC, SE, HA
Alyssum (P) Lobularia maritime N, SW, SC, SE, HA
Begonia (F) Begonia x tuberhybrida N, SW, SC, SE
Coleus (F) Coleus x hybridus N, SW, SC, SE
Fuschia (F) Fuschia x hybrida N, SW, SC, SE
Impatiens (F) Impatiens spp. N, SW, SC, SE
Nicotiana (Flowering Tobacco) (P) Nicotiana alata N, SW, SC, SE
Pansy (P) Viola spp. N, SW, SC, SE, HA
Snapdragon (P) Antirrhinum majus N, SW, SC, SE, HA
  • (P) = Tolerates dappled or part-shade conditions.
  • (F) = Tolerates full shade conditions.

Key to regional adaptation notes:

  • N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
  • SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello. SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
  • SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
  • HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Many annuals add scent to the landscape. The following annual plants provide not only the beauty of blooms, but provide pleasant fragrance for the garden. Try them in pots or beds near patios, balconies or other places you’re likely to frequent.

Common Name Scientific Name Regional Adaptation
Alyssum (P) Lobularia maritime N, SW, SC, SE, HA
Carnation (annual) Dianthus caryophyllus N, SW, SC, SE, HA
Mooonflower Ipomea alba N, SW, SC, SE
Nicotiana (Flowering Tobacco) Nicotiana alata N, SW, SC, SE
Pincushion Flower Scabiosa atropurpurea N, SW, SC, SE, HA
Scented Geranium Pelargonium sp. N, SW, SC, SE, HA
Sweet Pea Lathyrus oderata N, SW, SC, SE, HA
Sweet Sultan Centaurea moschata SW, SC
Verbena Verbena x hybrida SW, SC
Wallflower Cheiranthus cheiri N, SW, SC, SE, HA

Key to regional adaptation notes:

  • N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
  • SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello. SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
  • SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
  • HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Several species of annuals have attractive or interesting foliage that is of greater ornamental value than the flowers. These have a unique place in the landscape and container plantings and provide interest throughout the growing season, not just during the period of bloom. Most of these plants are tender annuals and grow best where the frost-free season is relatively long. Exceptions are ornamental kale and dusty miller.

Common Name Scientific Name Regional Adaptation
Amaranth, (Joseph’s Coat) Amaranthus spp. N, SW, SC, SE
Begonia Begonia x tuberhybrida N, SW, SC, SE
Caladium Caladium x hortulanium N, SW, SC, SE
Coleus Coleus x hybridus N, SW, SC, SE
Dusty Miller Senecio cineraria N, SW, SC, SE, HA
Geranium Pelargonium hortorum N, SW, SC, SE, HA
Impatiens Impatiens wallerana N, SW, SC, SE
Ornamental Kale Brassica oleracea N, SW, SC, SE, HA

Key to regional adaptation notes:

  • N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
  • SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello.
  • SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
  • SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
  • HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Although most vines are woody or perennial, a few annuals can also be used in situations best filled by plants that reach heights of at least four feet and self-attach. Here are a few that can be successfully grown in Idaho. Most adapt better to warm regions rather than colder, high elevations areas. There are exceptions, such as sweet peas.

Common Name Scientific Name Regional Adaptation
Black-eyed Susan Vine Thunbergia alata N, SW, SC, SE, HA
Canary Bird Flower Tropaeolum peregrinum N, SW, SC, SE
Cardinal Climber Quamoclit sloteri N, SW, SC, SE
Climbing Nasturtium Tropaeolum majus N, SW, SC, SE
Moonflower Ipomoea alba N, SW, SC, SE
Morning Glory* Ipomea purpurea N, SW, SC, SE
Scarlet Runner Bean Phaseolus coccineus SW, SC
Sweet Pea Lathyrus oderatus N, SW, SC, SE, HA

*Ipomea is not invasive like field bindweed, which is sometimes also called “Morning Glory.”

Key to regional adaptation notes:

  • N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
  • SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello.
  • SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
  • SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
  • HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Many annuals have appropriate form and flowering habit to enhance the appearance of hanging baskets or containers. This list is only the beginning. The use of baskets and pots can improve the appearance of barren area in the landscape, regardless of growing region. Click here to access more information on container gardening.

Common Name Scientific Name Regional Adaptation
Alyssum Lobularia maritime N, SW, SC, SE, HA
Black-eyed Susan Vine Thunbergia alata N, SW, SC, SE, HA
Dwarf Morning Glory Convolvulus tricolor SW, SC, SE
Coleus Coleus x hybridus N, SW, SC, SE
Fuschia Fuschia x hybrida N, SW, SC, SE
Impatiens Impatiens spp. N, SW, SC, SE
Ivy Geranium Pelargonium peltatum N, SW, SC, SE, HA
Lobelia Lobelia erinus N, SW, SC, SE, HA
Mandevilla Mandevilla splendens N, SW, SC, SE, HA
Million Bells Petunia Calibrachoa N, SW, SC, SE, HA
Periwinkle Catharanthus roseus N, SW, SC, SE, HA
Petunia Petunia spp. N, SW, SC, SE, HA
Sapphire Flower Browallia viscose N, SW, SC, SE, HA

Key to regional adaptation notes:

  • N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
  • SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello. SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
  • SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
  • HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Many annuals not only provide color in the garden, but look attractive and last well as cut flowers. Some, such as baby’s breath, cockscomb, globe amaranth, statice, bells of Ireland, and strawflower also make attractive dried arrangements.

For more information about growing cut flowers commercially, try the University of Idaho Extension publication Specialty Cut Flower Production.

Common Name Scientific Name Regional Adaptation
Amaranth, (Love Lies Bleeding) Amaranthus spp. N, SW, SC, SE, HA
Carnation (annual) Dianthus caryophyllus N, SW, SC, SE, HA
Baby’s Breath Gypsophila elegans N, SW, SC, SE, HA
Bells of Ireland Moluccella laevis N, SW, SC, SE, HA
Celosia (Cockscomb) Celosia cristata N, SW, SC, SE
China Aster Callistephus spp. N, SW, SC, SE, HA
Cornflower Centarea cyanus N, SW, SC, SE, HA
Cosmos Cosmos spp. N, SW, SC, SE, HA
Gaillardia Gaillardia spp. N, SW, SC, SE, HA
Globe Amaranth Gomphrena globosis N, SW, SC, SE, HA
Larkspur Delphinium ajacis N, SW, SC, SE, HA
Nasturtium Tropaeolum majus N, SW, SC, SE, HA
Marigold Tagetes spp. N, SW, SC, SE, HA
Salvia Salvia spp. N, SW, SC, SE, HA
Snapdragon Antirrhinum majus N, SW, SC, SE, HA
Statice Limonium sinuatum N, SW, SC, SE, HA
Sunflower Helianthus annuus N, SW, SC, SE, HA
Zinnia Zinnia elegans N, SW, SC, SE

Key to regional adaptation notes:
N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello.
SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Soil preparation is critical to success with annuals. Proper site and soil preparation will ensure a healthy environment for annual flowers. First, make sure the site has good quality topsoil. This may require addition of topsoil, particularly in new home sites. Amend the soil by adding 3-5 inches of well-aged compost or manure. This is especially important in the arid, calcareous soils of southern Idaho. Add the equivalent of 5 lb/1000 sq. ft. of nitrogen in the form of a complete fertilizer, such as 5-10-5. The fertilizer choice should be high in phosphorus and should preferably include sulfur. After amendments are added, the soil should be tilled to a depth of at least 8 inches, leveled, and smoothed (but not packed). Just prior too or after planting, it is a good idea to add two or three inches of mulch (wood chips, bark, etc.) to the soil surface.

Three options exist for starting annual plants. They are:

  1. direct seeding,
  2. indoor seeding and transplanting, and
  3. purchasing and transplanting bedding plants.
Direct Seeding

The simplest and cheapest propagation method is to plant seed directly into the garden site. A wider array of species and varieties are available as seed, providing the gardener with greater choice. The advantages of direct seeding are offset by the tendency for plants to be slow and erratic with respect to emergence and early growth. This may delay flowering and shorten the color display during the blooming period. This is especially true in the short growing season areas of Idaho’s mountainous regions.

Refer to the seed package for recommended date of planting. Plant seed where you would like the flowers to grow in the garden. Place seed in shallow trenches and cover lightly. Plant extra seed and thin after emergence, if necessary. Refer to the seed package again to determine seeding rate and depth. Maintain good moisture at the soil surface by misting lightly until the plants emerge. Once established, deeper irrigation should be applied after the top 1-2 in. of soil dries out.

Indoor Seeding

Growing your own transplants has the best features of both direct seeding and purchase of transplants. It gives the cost advantage and plant choice of direct seeding while making it possible to get a head start on the growing season.

The key to success is providing appropriate conditions for germination and early growth of the new seedlings. Soil, temperature, light, and moisture are the most important elements. The soil medium must be free of disease organisms that may cause death of germinating plants. The best soil medium is a commercial potting soil. Containers may range from recycled plastic pots, paper cups, or commercial seed plug trays. Wash and disinfect all containers before using. Temperatures should be warm enough to allow germination (e.g. 60-75º F during the day and slightly cooler at night).

Unless a greenhouse is available, artificial lighting will be required. Even a south facing window does not supply young plants with enough light to keep them from getting ‘leggy’ and weak. Some seedlings may need as much as 18 hours of light to be healthy. Fancy equipment and expensive “grow lights” are not necessary. A standard fluorescent shop light, easily found at home improvement and hardware stores, fitted with one “warm,” and one “cool” tube works very well. Suspend the light fixture 12-18 inches from the plants and raise as growth occurs. It is also important to keep soil moisture balanced between too wet and too dry. This required frequent, light irrigations.

Another important process in producing healthy transplants is called hardening off. This refers to the procedure of adapting the plants to outdoor conditions to reduce transplant shock. This can be done by placing the plants outdoors in full sun for increasing amounts of time each day for a week or ten days prior to planting. Hardening the plants will improve survival and increase the early growth rate. Those fortunate enough to have a cold frame or unheated greenhouse can use these structures for this purpose as well as starting seeds.

Purchasing Bedding Plants

Buying partially-grown plants is the easiest and quickest way to establish annual flowers. It is also the most expensive and provides the least in the way of plant choice. But, transplants will result in quicker blooms and longer flowering periods.

When choosing transplants, it is best to buy from a nursery or garden store having personnel knowledgeable about local growing conditions. This will assure availability of adapted species and varieties. Do not look for the largest plants or necessarily, those in bloom. Seek plants with good dark green color, healthy root systems, and no sign of disease or pest problems. Try to find plants that have been hardened off, in order to aid the transition to the yard.

Another important consideration for growing transplants is the timing initial planting. To properly make this decision for a particular flower, it is essential to know the amount of time needed to produce a transplant and the approximate date of intended transplanting outside.

Transplanting

Whether transplants are purchased or self-grown, the process for placing them outside is the same. The first decision is deciding when to place the plants outdoors. Transplanting date is based on the date of last frost in a given area. Tender annuals should not be planted to the garden until 1 to 2 weeks after the average last frost date. It is, after all, an average and frost will commonly occur after the printed date.

To estimate the last frost date in your area, look at the Idaho chart compiled by Ed Hume Seeds.

Tonie Fitzgerald, from the Spokane, Washington County Extension office compiled a table listing appropriate dates for planting and/or transplanting common annual flowers. The dates should be reasonably accurate for much of northern and south-central Idaho. Dates for the Treasure Valley of southwestern Idaho will be 2-3 weeks earlier, southeastern Idaho a few days later, and the high country up to 2 weeks later.

frost damage

Frost can damage seedlings or transplants

It is best to transplant on a cool, cloudy day with little wind. This will allow acclimation under conditions of limited water loss. After removing a plant from its container, tease roots away from the surface of the root ball. Don’t plant the seedlings too deep. Bury the root ball in a hole sufficiently deep only to bring the soil slightly above the pot soil level. Space the plants according to the instruction on the seed packet or nursery pot label. For the first 7-10 days, water the plants frequently and lightly. Remember that early on the pot soil holds all of the roots and is the only source of water. The root ball will need to be wetted as often as it would in the pot until the roots can grow into the surrounding soil.

Once established, many annual plants are relatively carefree. However, as is true of all plants, some tender loving care is needed to keep them healthy and attractive.

mulch

Mulch provides many benefits

Mulching

If not done before planting, it is beneficial to mulch the flower bed before heat of summer sets in. This will keep the soil cool, retain moisture, and help with weed control.

Irrigation

Annual flowers use about the same amount of water as does lawn, equal to about ¼ in. per day during July and August. In most Idaho soils, this means approximately 1 in. of water should be applied every 4 to 5 days. In sandy soils, less water should be applied on a more frequent basis. The amount of water applied should be cut back during the cooler spring months, the late fall, and during those infrequent periods of rain. The simplest method of judging water need is to wait between irrigations for the top 1-2 in. of soil to dry.

Fertilization

In most loam soils, preplant fertilization may be adequate to provide basic nutrient needs for annual plants. However, under conditions of sandy soils or long growing season, there may be benefit in adding a small amount (equivalent of 2-3 lb nitrogen per 1000 sq. ft.) of a fertilizer that is high in nitrogen. Sprinkle the fertilizer on the soil surface and water it in. Make the application in late June or about the time the flowers reach peak bloom.

Manicuring

Some annuals need a little attention to appearance to remain attractive. Plants that look thin and leggy can be forced to produce more lateral growth by shearing or pinching off the growing point of each stem. Plants that fall down or become floppy may need to be staked or interplanted with stiffer, more upright types of plants. Many annuals either do not shed dead flowers or produce seed heads, thus reducing production of additional flowers and making the plants less attractive as the summer progresses. This can be solved by occasionally removing the dead flowers, a practice called “deadheading”.

seed head

Removing old seed heads encourages flowering

Weed Control

There are no control options that completely replace hand weeding in annuals. Mulching with organic matter or weed barriers will help by blocking germination and growth of weed seed. Some partially effective herbicides are available to help with weed control in annuals. All of these must be applied after the flowers emerge or are transplanted, but before the weeds emerge. See your county agent or local nurseryman for information on products available.

Disease and Insect Control

With over a hundred species of annual flowers commonly available, it is beyond the scope of this site to provide specific pest management information for each one. However, there are many pests that are common and infest many types of plants. We have compiled information on the most common of these in our pages on insect and disease problems.

For more detail on control of insects and diseases, as well as information of other pests not covered in the sections above, see the Insect and Disease Pests section of this site.

Aphids

Also known as plant lice, aphids are small, soft-bodied, sucking insects that cluster on the stems or underside of leaves. Aphids are usually wingless and green, brown, or black in color. Symptoms of infested plants include distorted or curled leaves, presence of sticky sap (honeydew) on the infested surfaces, and misshapen new growth.

Aphids can be controlled with the use of insecticidal soap or a registered insecticide. A strong stream of water directed at the infected plants may knock aphids from the plant. Many beneficial insects such as ladybeetles and lacewings feed on aphids and if an infestation is not too severe, it may be appropriate to be patient and let nature take its course.

Caterpillars

Caterpillars can quickly damage annual plants

Caterpillars

Larvae of numerous species of moths and butterflies. These voracious creatures come in many sizes and colors. Plant symptoms include chewed or completely missing leaves. Some types of caterpillars will roll or fold the leaves and hide inside. Often, frass (droppings) are present on and around the plants.

A light infestation can be easily controlled by picking larvae from the plant and crushing them. Common registered insecticides will effectively kill caterpillars.

Grubs and Cutworms

Grubs and cutworms are the larval stage of many moths and beetles. Most live in the soil and feed on roots or emerge at night to feed on stems and foliage. Severely damaged plants may die from having the stem or roots severed. Other symptoms include chewed lower leaves and/or wilted or stunted plants that result from root feeding.

Grubs and cutworms can be controlled by handpicking, or using a soil drench of an approved insecticide.

Leafminers

Leafminers are small insect larvae that burrow under the leaf surface while feeding. Symptoms are easily recognized and exhibit themselves as zig-zag or wandering lines on the upper leaf surface that are lighter in color that the rest of the leaf surface. These are tunnels in the leaves caused by leafminer feeding.

A light infestation of leafminers can be controlled by removing and destroying damaged leaves. A heavy infestation will require the use of a registered systemic type insecticide.

Sometimes covering a crop with a floating row cover will exclude the insects from entering the plant.

Mealybugs

Mealybugs are sucking insects that infest stems of many plants. Mealybugs are easily recognized by the presence of a cotton-like white substance they deposit for protection.

Control of mealybugs can be had by spraying the plants with a direct stream of water, using an insecticidal soap, or applying a registered insecticide.

Spider Mites

Not actually insects, these miniscule pests are related to spiders. They spin protective webs on the underside of leaves and feed by sucking juice from the leaves. Symptoms include color mottling that, at a distance, may appear as a general yellowing of older leaves. Webbing will be present on the underside of infested leaves. The mites, to small to be easily visible, can be detected by shaking a leaf over piece of clean white paper.

Spider mites prefer dry, dusty environments. Sprinkler irrigation or routine washing of leaves with water usually keep them at bay. A severe infestation may require the use of a registered miticide. Most common insecticides are ineffective against spider mites.

slug

Slugs prefer damp places in the garden

Slugs and Snails

Slugs and snails prefer damp soil and humid conditions. Slugs and snails often hide during the day and feed at night. Symptoms include chewed leafs and glistening slime trails on plant surfaces.

Control snails and slugs with baits.

Thrips

Damage is caused by the larva of this small, four-winged insect. Thrips reside on the underside of leaves and use their rasping mouthparts to scrape away the surface of the leaf after which they feed on the sap. Symptoms appear as small white streaks and blotches, more prominent on the underside of the leaf.

A light infestation does little permanent damage to the plant and can be ignored. A heavy infestation will likely require the use of a registered insecticide.

Whiteflies

In Idaho, whiteflies are more commonly a problem in greenhouses than they are outdoors. They are small insects with distinct bright white wings that reside and feed on the underside of leaves. Symptoms include the presence of honeydew on leaf surfaces, often accompanied by a black sooty mold. When disturbed, clouds of the white, rapidly flying insects will rise above the foliage, then quickly resettle.

Trap the flies with yellow sticky boards or use a registered insecticide.

Aug162012
Damping off

Damping off disease of seedlings © 2004 Cornell Plant Pathology Herbarium

Damping Off

Damping off is caused by fungal pathogens that infect seedlings at soil level, girdling the stems and causing death. Infected seedling will develop tan-colored, soft tissue at the base of the stem. The plants fall over and usually die. Once established and actively growing, plants are seldom affected by damping off. Control measures include maintaining optimum soil moisture and planting into well-drained soils that are not overly wet. Maintaining air circulation is important. In extreme cases, a soil drench of a registered fungicide can be applied to the soil surface. However, by the time damage is observed it may be too late for effective control.

Leaf Spots

Leaf spots are caused by numerous fungal (occasionally bacterial) pathogens that penetrate and kill leaf tissue. Symptoms usually start and are worse on older leaves. These diseases are usually worse following periods of wet weather and high humidity.

Removal of all dead plant material at the end of the growing season helps prevent many leaf spot diseases the following year. In-season control usually requires use of a registered fungicide.

Powdery Mildew

Powdery mildew is caused by fungal pathogens. The classic symptom is a whitish, powdery growth present on leaf surfaces. Heavy infections cause distortions on new growth. Infections are often worse during summers preceded by damp spring weather. Plants grown in shade are more prone to infection with powdery mildew

Prevention involves growing plants in a sunny location and making sure there is plenty of space and air movement around plants. Control of severe infections may require the use of a registered fungicide.

Root and Stem Rots

Root and stem rots are caused by fungi (occasionally bacteria) that live in the soil. Infected plants initially develop mild wilting symptoms that become progressively worse and may eventually cause death.

Soil pathogens are difficult to control. They can best be prevented by planting resistant varieties, avoiding overly wet soil conditions, and destroying infected plants.

White Mold

White mold is caused by a fungus that overwinters in the soil. It infects plant stems that touch moist soil surfaces. Symptoms include a slimy, white mold that girdles and collapses the infected tissue. Leaves above the girdled stem wilt and die. In advanced stages, small gray structures that look like mouse droppings form inside a hollowed stem.

Prevention is the best strategy and involves staking stems off the ground, spacing plants to allow air movement around foliage, and irrigating infrequently to allow intermittent drying of the soil surface.

Virus

Not technically alive, viruses are small pieces of genetic material that disrupt plant function. Symptoms vary widely and usually include some combination of stunting, yellowing, mottling, or leaf and stem distortion.

Consistently effective control measures for viruses are rare. The best methods include using resistant varieties or preventing the infestation of organisms (usually insects) that transfer the viruses from one plant to another. Prevention also involves removing and destroying any infected plants.

For a more complete discussion of care for annual flowers, including selection, planting, general management, and pest control, see chapter 19 of the Idaho Master Gardener Handbook.

Additional information on insect and disease management in annuals is provided in this list of publications from the University of Georgia.

Diagnosis information and specific control measures for diseases in the landscape is available from the University of Kentucky.

University of Idaho Extension

Physical Address:
E. J. Iddings Agricultural Science Laboratory, Room 52
606 S Rayburn St.
Moscow, ID

Mailing Address:
University of Idaho Extension
875 Perimeter Drive MS 2338
Moscow, ID 83844-2338

Phone: 208-885-5883

Fax: 208-885-6654

Email: extension@uidaho.edu

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Barbara Petty