Trees and Shrubs in the April Garden

• Trees help counteract the "urban heat island" effect. Urban areas with a high percentage of concrete, highways, glass, and other objects are hotter than rural areas. These urban heat islands have reduced ventilation and warm temperatures. This causes an increase in the amount of ozone pollution in the air. Rural areas with trees are less likely to have the urban heat island effect, resulting in less ozone pollution.
• Don't coat pruning cuts with tree paint or wound dressing, except for control of certain disease-carrying insects. These materials won't prevent decay or promote wound closure. Some tests, however, have shown wound dressings to be beneficial on trees that are susceptible to canker or systemic disease.
• Not all plants are harmed by juglone (the toxin given off by black walnut trees), and even the most susceptible are sometimes only slightly affected. This reduced effect is explained by research at the University of Colorado, showing that where drainage is good, juglone is decreased or inactivated. Soil with good aeration contains certain species of Pseudomonas bacteria that feed on juglone. These bacteria are absent in heavy and wet soils with limited oxygen.
• Layering has been found to be successful on more species of trees and shrubs than any other style of vegetative propagation. Layering consists of wounding a branch of the plant, then covering the wounded area with a rooting medium, such as soil or sphagnum moss. The branch usually will form roots around the wound while it is still attached to the parent plant. Layering is most successful if done in spring or late fall as rooting is most vigorous in cool weather.
• Hydrangea is one gift plant that transplants well into the garden after its flowers fade. When the weather warms, plant in well-drained soil in full sun to partial shade. Don't be surprised if the next year's flowers are a different color than the first year. Blue or pink hydrangea color is dependent on the pH of the soil. Alkaline soil produces pink flowers; acidic soil produces blue flowers. White hydrangeas are not affected by soil pH.
• Many gardeners plant annual and perennial flowers to attract hummingbirds. Woody plants can also be added to the yard to provide nectar for our smallest native birds. Some common trees visited by hummingbirds are buckeye, horse chestnut, catalpa, apple, crabapple, hawthorn, silk tree, redbud and tulip poplar. Shrubs include azalea, beauty bush, coralberry, honeysuckle, lilac, New Jersey tea, Siberian pea shrub and red weigela.
• Heavenly bamboo nandina is a frequently used shrub. Consider opting for a unique variety such as 'Harbour Dwarf' with foliage ranging in height from 18 to 24 inches. It spreads by underground rhizomes, and with its red, winter color and interesting texture, makes an excellent, low-maintenance, ground cover.
• Don't add organic matter to the soil when planting trees. It does not help the tree become established and may create conditions that encourage the roots to stay inside the planting hole instead of spreading into the surrounding soil. Do dig a large planting hole, but fill it with the original soil that was removed from it.
• For more-compact, pyracantha bushes without the risk of losing the crop of berries, pinch back new growth now.
• The last Friday in April is National Arbor Day - plant a tree or support an organization that does!
• Some shrubs grow best on acid soils with a pH of about 5. These include andromeda, azalea, blueberry, camellia, mountain laurel and rhododendron. At a higher pH value, these shrubs may become yellow and have very poor growth.
• Prune roses to buds that point outward to encourage good air and sunlight penetration. Dark-colored canes indicate dead wood. Cut back an inch below these darkened areas. If no live buds are left, remove the entire cane or branch.
• Prune spring-blooming shrubs, such as forsythia, weigela and early spirea, after they have completed flowering.
• If dogwood leaves have been small, sparse and pale, the trees may need fertilizer. Take a soil sample from the area beneath the trees using instructions provided by your Extension agent. Return the soil sample to the Extension office and request a soil test. Correct fertilizer recommendations will be returned with the test results.
• Once new growth begins on trees and shrubs, cut back to green wood any twigs affected by winter kill.
• Aphids on your roses will cause deformed, inferior flowers. If aphids are noticed, contact your Extension agent for an approved insecticide that you can apply.
• Tree seedlings can have a strong start in quart or half-gallon milk cartons. Remove top and bottom and stand on a frame covered with 1/4-inch, mesh hardware. Fill with a mixture of peat moss, perlite and vermiculite. As the tap root grows through the bottom of the milk carton tube and mesh, it is "air pruned" forcing the development of fibrous, side roots.
• To repair damaged tree leaders, a lateral or side branch can be trained while still young and flexible to become a strong, terminal leader.
• Do not fertilize azaleas and camellias until they have finished blooming. They also should be pruned after blooming.
• The best time to plant shrubs and trees is on a windless, cloudy day.
• When pruning forsythia, do not shear as you would a hedge. It is best to thin out the old branches as close to the ground as possible. This should be done immediately after blooming.
• Before planting bare-root shrubs and trees, soak the roots in water overnight.
• If wisteria does not bloom, it needs careful pruning to correct the condition. Prune long, straggling canes and all dead wood. Root pruning sometimes helps, too.

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