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Idaho Landscapes and Gardens

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Landscapes and Gardens

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Trees, Shrubs and Vines

Trees, shrubs and vines serve many purposes and offer numerous benefits including shade, screening for privacy and windbreaks, and noise reduction. They also provide shelter for a variety of wildlife. Groundcovers can visually define a space or provide texture and color to a landscape. In this section, you’ll find useful information on how to select and care for your valuable landscaping plants.

Fertilizing

The type of soil that a tree or shrub grows in can affect its nutrient needs. Soil texture and soil structure influence the amount of water, air, and nutrients held in the soil for plant use. Clay soils can be nutrient rich, but have a large amount of fine particles that tend to compact and restrict water and air movement. Sandy soils drain well, but contain many coarse particles that have little capacity for storing water, air and nutrients. Organic material can be thoroughly mixed into soils with high clay or sand contents to help improve soil structure. Repeated applications may be needed depending on the amount applied and the stage of decomposition or type of organic matter used. Organic material should be mixed into the soil up to several years before trees are installed to obtain maximum benefit.

No single symptom tells you that trees or shrubs need additional fertilization. Some nutrient deficiency symptoms can be similar to symptoms of cultural problems or diseases. Slow growth rate, small leaves, fewer flowers, smaller fruit, and pale green or yellow (chlorotic) foliage with mottling between the leaf veins may all be signs of nutrient deficiency.

Two methods of determining nutrient deficiencies include:

Soil Testing

  • Advantages — provides soil pH, levels of K, P, organic matter content and minor nutrients such as iron or zinc.
  • Disadvantage — does not provide reliable information on N because N is rapidly lost through leaching or removed by plants

Plant Analysis

There are two methods of determining nutrient deficiencies through plant analysis:

Visual symptoms — include length of shoot growth, leaf color, leaf size, and color pattern and timing of leaf drop

  • Advantages — N and Fe are often the easiest visual symptoms to identify
  • Disadvantage- symptoms can be deceiving and/or nonspecific

Foliar tissue analysis — provides the concentrations of specific elements in plant foliage (usually leaves)

  • Advantages — when combined with soil tests it can provide a good picture of nutrient problem(s) — deficiency or toxicity
  • Disadvantage — nutritional needs for many landscape plants is unknown

Trees should be fertilized in early spring or mid-fall as long as the soil temperature is above 40º F two inches below the soil surface. Soil should also be moist. Avoid fertilizing in late summer and early fall as a nutrient application at this time could cause unwanted succulent growth that may fail to harden off before fall frosts hit.

Fertilizer application methods

Broadcast or topdress — fertilizer is added directly to the soil surface. This method is good for N, which moves readily through the soil, but poor for P and K that move slowly through the soil. Fertilizer should be applied to the drip line and at several foot intervals out from the drip line for mature trees.

Soil incorporated — dry or liquid fertilizer is added to holes in the soil beneath the canopy and extended beyond the drip line and provide a long lasting effect. Holes should be up to 12 inches deep and 1 to 2 inches in diameter and made in concentric circles 2 feet apart around the tree trunk with the first circle no closer than 3 feet from the trunk.

Foliar sprays — best for supply nutrients for plant use in only trace amounts, such as Zn, Mn and Fe.

Tree spikes are a dry soil injection method, with a hardened column or cylinder of fertilizer hammered into the soil.

Controlled release pellets are typically broadcast on the soil surface, but they can also be placed in holes augured into the soil.

Tree spikes and slow release pellets may delay the development of winter hardiness so it is best to use them in late fall or early spring.

Recommended rates of fertilizer are calculated using the ground area under the tree canopy. The amount of fertilizer to add depends on the fertilizer composition and is usually calculated using the desired N rates. Nitrogen rates range from 0.2 to 0.4 pounds per 100 ft². Excess nitrogen can be detrimental to plant growth.

Quantities of common fertilizers, incorporated into the soil, needed to provide equivalent amounts of N.

Applications rates expressed pounds of fertilizer material per 100 ft² of ground area.

Fertilizer Rate
Fertilizer analysis 0.4 lb N per 100 ft² 0.25 lb N per 100 ft²
2-3-2 20 lbs 12.5 lbs
4-10-4 10 6.2
6-10-4 6.8 4.2
8-10-8 4.8 3.0
10-10-5 4.0 2.5
12-12-5 3.2 2.0
14-10-0 2.4 1.5
16-20-0 2.2 1.4
20-16-0 2.0 1.3
21-0-0 2.0 1.3
33-0-0 1.2 0.8
45-0-0 0.8 0.5

from 1997 Bulletin CIS 1068. Fertilizing Landscape Trees

Sample Fertilizer Application Problem

If a 10-year-old tree has a canopy that is 20 feet wide, a trunk that is 5 inches in diameter, and roots that extend 35 feet from (one side of) the trunk, how much 18-6-12 fertilizer should be applied via the broadcast method? Use a rate of 3.5 lb. of nitrogen (N)/1000 ft²

First, figure the surface area occupied by the tree’s roots. Area =π*r² where π = 3.14 and r = 35 ft.
= 3.14 x 352
= 3846.5 ft2

Second, calculate the number of pounds of nitrogen needed to cover the root zone.
Pounds of Nitrogen = 3.5 lb. N x 3846.5 ft²
Needed 1000 ft²
= 13.5 lb. N

Third, calculate the number of pounds of fertilizer needed to cover the root zone.
1. Important Relationship
Pounds of Fertilizer x Percent Nutrient = Pounds of Nutrient

2. Calculation for
Pounds of Fertilizer 13.5 lb. N
Needed    = 0.18 N     = 75 lb

Planting

Early spring and early fall are the best times of the year to plant because plant shoot growth is minimal and roots have time to become established after planting. Bare root plants should be planted before bud break in March, April or May. Balled and burlapped and container plants can be planted anytime of the year as long as the soil is not frozen. However, early spring or early fall are still considered the best times to install these types of nursery plants.

Where to Plant

Select plants appropriate for the location in which they’ll be planted. Pay attention to the eventual mature height and spread of a tree or shrub, keeping in mind that some community ordinances may restrict planting of trees near power lines, parking strips, street lights, sewers, traffic control signs and signals, sidewalks and property lines.

Other questions to consider

  • Will this tree or shrub drop leaves, flowers, or fruit that may be a nuisance to neighbors?
  • Will this plant receive the sufficient amount of sunlight in this location? Will it shade other plants?
  • Will this plant share moisture requirements with the plants surrounding it? Is it compatible?
  • What kind of care, including pruning, will this plant require?

Many of the resources listed here provide information to help homeowners answer these questions.

Bare root plants should be unpacked carefully and any broken roots should be cleanly pruned off. The roots can be soaked in a bucket of water for up to 4 hours and protected from heat and drying before planting. Do not prune any healthy roots now. Once the hole is dug and the roots spread out, any roots that are too long may be removed at that time to prevent circling or kinking.

Balled and burlapped plants should be handled carefully by the soil ball, not the trunk or branches, to prevent root and trunk damage. Once the plant is in the hole, twine, nails and excess burlap should be removed from the top of the root ball. Synthetic burlap and in-ground fabric bags should be completely removed.

Container plants should be kept in their containers until they are ready to be put in the ground to protect the roots from drying out or getting damaged. Fiber and pulp pots decompose slowly and should be removed completely or sliced before planting to avoid rot and drainage problems. Some plants may be pot bound or have roots that are matted together. To improve root growth into the surrounding soil make 4 to 5 shallow cuts around the root ball and loosen the roots on the sides and bottom of the root ball before planting. For severely pot bound plants, the butterfly method can be used.

Dig the hole only deep enough to hold the root ball. When planting in loam soil, the root flare on the tree or shrub trunk is planted level with the surrounding soil. If you can’t see the root flare, remove soil or burlap until you do. When planting in clay soil, the root flare is planted 1 to 2 inches higher than the surrounding soil (see diagram). Alternatively, you can form a one-inch pedestal at the bottom of the planting hole for the root ball.

planting depth

The planting hole diameter should be two times the diameter of the root ball. The minimum planting hole diameter can be 12 inches wider than the root system and the maximum can be up to five times the root system diameter. The sides of the planting hole should be vertical. If the sides of the planting hole appear shiny or glazed, rough up the edges with a shovel to loosen the soil before planting.

Before adding soil back into the planting hole, make sure that roots are not kinked or circling. Start adding soil in three to four inch layers lightly firming the soil between layers by lightly stepping on the soil. Backfill soil may be mixed with organic matter at a rate of 3 parts soil to 1 part organic matter to help improve soil texture of medium or fine textured soil. Adding organic matter to the backfill soil is not always recommended, especially in a heavy clay, since sometimes roots will not leave a richly amended hole to grow into the adjacent inferior soil, and circling of roots may occur. Avoid covering the top of the root ball with backfill soil. Make a 2 to 3 inch soil berm around the edge of the planting hole to form a shallow basin. Water the plant in well by filling the basin with water. In heavy clay soils, watch that this berm drains within a couple of hours. Fertilization is not recommended at time of planting. Once the plant is well watered in, apply a layer of mulch 2 to 3 inches thick to the basin, avoiding placing mulch against the base of the trunk.

Pruning

Selecting the proper time to prune is important. Heavy pruning at the wrong time of year can stimulate unwanted growth or prevent flowering or fruiting. Before pruning, consider time of year, type of plant and flowering periods of certain plants. See the table below.

Time of Year to Prune Various Types of Plants.
Season
Type of plant Fall Winter Spring Summer Comments

Early Late Early Late Early Late Early Late
Deciduous shrubs







for shrubs
that flower before
June 1
Deciduous shrubs







for shrubs
that flower after
May 30
Deciduous
trees
a







Conifers –
Shrubs and
Trees







All conifers
except for pines
(see below)
a
Pines
a







Broadleaf
evergreen
shrubs







For shrubs grown
for flowers
Broad leaf
evergreen
shrubs







For shrubs grown for foliage (hedge)

When to prune new growth on pines

Pines have buds only at the tip of the branches. If a branch is pruned after a growth flush and the terminal bud is removed, regrowth is impossible. Pine branches should be pruned or pinched in early summer when the new branch (candle) has begun to elongate but before the needle bundles open. This pruning causes the growth to be more compact but still allows buds to form for the following year.

  • Heading Cut — Cutting plant stems back to a bud, twig or stub. Potential problems — a stub is often left and may become infested with insects or diseases, vigorous growth may be stimulated, and the new growth may be weakly attached and could split or crack under pressure. This may also negatively affect the desirable, graceful arching habit of some shrubs.

Heading cut

  • Thinning Cut — The removal of a branch at its point of origin or cutting back a branch to a lateral branch about 1/3 the diameter of the branch being removed. Advantages — No stub is left, the plant retains its natural shape, and vigorous new shoot growth is avoided. Caution: removing more than about 30% of the foliage can stimulate new growth even if thinning cuts are used.

Thinning cut

Using thinning cuts on selected branches to improve light and air penetration to interior foliage.

The size of the branch determines the location of the pruning cut. A small twig or branch can be pruned back to a bud or lateral branch. On a small branch make the cut one-quarter inch above the bud, and slant the cut away from the bud (see diagram).

Length of pruned stub

  • Natural target pruning is the technique used when cutting beyond the bark ridge and branch collar (collectively called the branch shoulder). see diagram below. This cut provides a physical barrier to disease and injury within the intact branch shoulder. This compartmentalization of the wound stops injury from spreading into the main trunk of the plant where it could spread throughout the plant and be damaging to the plant as a whole. The wound size is also smaller allowing callus to grow quickly over the wound and limits potential exposure to insect and disease infestation.

Natural target pruning

  • Large branches or limbs (those to heavy to hold while cutting) are removed by a series of three cuts. The first cut is made on the underside of the limb about 12 inches from the branch crotch and should go about one -quarter of the way through the limb. The second cut is made on the top side of the branch about 2 inches farther out on the limb from the undercut. Cut down until the branch cracks off. As you are completing the second cut, careful to watch for the branch suddenly dropping or moving quickly as the branch weight accelerates limb removal. The third cut is made on the top side of the stump left over, just outside of the bark ridge and branch collar. See the diagram (on next page) to determine where the final pruning cut should be made. Be sure to support the branch stump when making the final cut to avoid tearing the tissue when completing the cut.

Three cut method diagram for pruning large branches:

Three-cut method for pruning

Proper tools should be used to make a clean pruning cut and to minimize damaging plant tissue. If the plant tissue is crushed or torn it can leave the plant susceptible to disease and insect problems. In addition, more time will be needed by the plant to have tissues grow over the wound. Pruning tools should be the correct size for the job and be made of tempered steel that can hold a sharp edge. Making a pruning cut should be relatively easy when the correct size of tool is used. Hand pruners should be used to cut branches that are less than one-half inch in diameter. Lopping shears should be used for branches between one-half and 1 inch thick. A bow saw or pruning saw should be used to cut branches larger than 1 inch thick. Be sure to make a clean cut with the proper tool.

Sanitizing pruning tools

Pruning tools should be disinfected after each cut, under ideal circumstances, to avoid spreading diseases from plant to plant. At the very minimum disinfect pruning tools after finishing one plant but before beginning to prune the next plant. To sanitize tools, dip the cutting edge in a disinfectant solution such as denatured alcohol, methanol or diluted household bleach (1 part bleach plus 9 parts of water). An alternative is to spray the cutting blade with a disinfectant solution. When using bleach, make sure to apply a thin layer of oil to the blade before storing to avoid rusting of the tool.

Pruning paints and asphalt emulsions are not recommended for use on pruning cuts as they may actually seal in disease-causing organisms or promote rot.

  • How to prune coniferous evergreen trees BUL 644
  • How to prune deciduous landscape trees BUL 819

Other websites that present pruning information are:

Hydrangea Pruning
Pruning Peach Trees

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Landscapes and Gardens

University of Idaho Extension

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