|This Week’s Garden Tip 08/15/14
Christian County Extension Office
By determining potential landscape disease problems prior to planting, landscape plant health and longevity can be improved.
Too often perfectly good plants are located in sites that favor disease outbreaks. An important preventive concept is to choose plants that grow well in a particular landscape site, or else modify the site so plants will grow at their best. If plants must struggle to overcome the effects of a poor growing location, they are more prone to diseases. Rhododendrons and azaleas, for example, require well-drained acid soil. If the soil is poorly drained, they are vulnerable to root rot, and if the soil is not acidic, they will turn yellow from iron deficiency. When the landscape site is not naturally suitable, you should modify the site by improving drainage and lower the pH prior to planting to avoid these problems or else consider a plant that will grow in the site. Similarly, white pines can grow well in some Kentucky locations, but they need acid and sandy soil. If the roots are not provided with these good conditions, expect white pine decline to gradually take out the trees. Yews (Taxus) are another common shrub that need well-drained soil to avoid root rot, but unlike other evergreens, they perform poorly in acid soil.
If landscape plants such as crabapples, dogwoods, roses, and other plants prone to leaf diseases are planted in shady locations or if they are planted so densely that good air movement and ventilation are not possible, then the planting site has been poorly selected. In such cases, expect outbreaks of diseases such as scab, black spot or powdery mildew when susceptible cultivars are planted.
When it is available, disease resistance is the most efficient means of plant disease control. When it comes to fighting diseases of landscape plants, it sometimes pays to know the name of the cultivar and how it is likely to respond to diseases that are common here. For example, flowering crabapple is a popular small tree for residential landscapes. With flowers, foliage and fruit, it can be attractive in all seasons, especially during bloom. Unfortunately, it can also distract from the beauty of the landscape when it is diseased. Flowering crabapple has many disease problems causing spotted, wilted leaves, rotted fruit, and premature leaf fall. But these diseases could be avoided by planting cultivars that are disease-resistant such as ‘Prairiefire’, ‘Mary Potter’, and ‘Harvest Gold’.
There are many other disease resistant trees and shrubs. There are elm cultivars resistant to Dutch elm disease, roses resistant to black spot, rhododendrons resistant to Phytophthora wilt, tip blight resistant junipers, rust resistant hawthorns, and the list goes on. Incorporating disease resistance into the garden is good preventative plant health care.
Finally, how a tree or shrub is planted makes an impact on its health and consequently its susceptibility to disease. Planting done right will pay big dividends later. It all starts with the hole. The planting hole needs to be wider at the top sloping downward to a depth equal the height of the rootball, specifically to the point where the tree trunk becomes a root (called the trunk flare). So many landscape issues could be avoided if we could only get this step right. The trunk flare should be at or slightly above the soil surface. If you can’t see the trunk flare in the container or rootball remove soil until you do and then dig your hole accordingly. Backfill your hole with the same soil that came out and water to remove air pockets. If you notice any roots starting to circle the trunk, cut them out. In a few years, these circling roots will connect with the trunk and cause girdling of the tree, leading to decline and eventual death. Also remember newly planted trees and shrubs are under stress and nothing helps stressed trees better than sufficient water.
To maintain long-term tree and shrub health in the landscape, make good decisions now. Choose the right location for new plants in the landscape; choose disease-resistant plants; and plant trees and shrubs properly in order to start them off right and to assure they will have good health for a long time.
For more information on gardening, you can call the Christian County Cooperative Extension Service at 270-886-6328. With this week’s garden tip, I’m Kelly Jackson.
Educational programs of Kentucky Cooperative Extension serve all people regardless of race, color, age, sex, religion, disability, or national origin.
UNIVERSITY OF KENTUCKY, KENTUCKY STATE UNIVERSITY, U.S. DEPARTMENT OF AGRICULTURE, AND KENTUCKY COUNTIES, COOPERATING