Flowering DogwoodsDogwoods are small ornamental deciduous trees used in residential and commercial landscaping. Valued for their spring flowers, summer and fall foliage, and fruit, Cornus landscape varieties are essentially composed of several cultivars of Cornus florida and Cornus kousa and a limited number of Cornus nuttallii cultivars. Colors can range from white to pink to even reddish. The hardiness is generally limited to Zones 5 through 8 and the typical size is 20 to 25 feet tall with comparable spread. All dogwoods enjoy partial shade and well-drained, rich soil.
What kind of Dogwoods are there?
Cornus florida 'Rubra' is the standard Pink Dogwood that is found across the United States.
Cornus florida refers to the original native White Dogwood species of United States. It has a multitude of cultivars and varieties and is naturally a white flowering shrubby tree. In all varieties, its flowers occur before the leaves come out in the spring.
Regretfully, the native dogwoods are plagued by several diseases and insects when stressed and require special care.
Cornus kousa is the native dogwood of Korea and Japan. Unlike the United States dogwoods, Kousa Dogwood flowers after the leaves have started to emerge. A more shrubby tree, there are not nearly as many cultivars because it is quite a bit newer to American gardens. It is also more resilient to insect and disease than the American dogwoods.
Cornus nuttallii is also grown to a lesser extent primarily in the Pacific Northwest and shares the same susceptibilities that Cornus florida is prone to.
*Furthermore, there are several Cornus florida x Cornus kousa hybrids available to the public which are more disease resistant to common dogwood diseases.
What kind of flowers can I expect?
Many people are unaware that Dogwood flowers are not really true flowers! The main reason
Dogwood blooms last as long as they do is because the colored portion
that you see is not a flower petal, but instead a bract. These bracts are modified leaves and the true flowers are actually very small and uninteresting. They are visible right between the colored bracts. Because of the shear number of available cultivars on the market, we typically advise you pick out a tree in bloom in order to make sure that you
get the color you truly want. While generic varieties, specifically
Cornus florida ‘Rubra’, can vary widely in flower color, the specific named varieties tend to be more uniform because they do not rely on seed origin and many other nonspecific factors.
Sources of Plants
Dogwoods can be found widely at most sites throughout the Eastern United States. It is always best to buy stock from local nurseries to ensure that your tree(s) are adequately adapted to your zone. Trees purchased from nurseries further south, tend to be less hardy for northern environments and can either die or produce less than spectacular spring flowers. Be sure to avoid trees with broken or dead branches, trunk damage, or leaf spotting. Trees with these symptoms indicate unhealthy or stressed trees. Digging dogwoods from surrounding woodlands is not advisible. Diseases and insect infestation can be inadvertantly transmitted to any existing healthy trees.
Choosing a Proper Planting Site
Because dogwoods naturally grow as understory trees in woodlands and forests where organic material naturally forms a surface mulch, it is always best to choose a location that mimmicks these conditions with partial shade and rich fertile soil. Dogwoods planted in poorly drained areas or areas with full sun will typically become problem ridden, have slow growth, and are more likely to eventually perish before maturity.
Dogwoods are shallow rooted trees. This means that special attention should be given when planting so that the dogwood is not planted too deep. Like all trees and shrubs, dogwoods typically respond best when planted during the late fall and winter while the tree is dormant. For full instructions on planting procedure including planting depth, technique, and specific container instruction, please visit boydnursery.net/planting/.
Maintenance of Established Trees
Healthy dogwoods result from an established maintenance routine. By watering, fertilizing and mulching existing trees, any home owner can successfully grow and care for their dogwoods. When rainfall is irregular during the summer and fall months, dogwoods require supplemental watering. While watering in the morning is always a best practice for trees in general, dogwoods are best watered by carefully soaking the base of the tree. This practice helps both because wetting the foliage can encourage diseases and because dogwoods have naturally shallow root systems. The soil should be watered to a minimum depth of 6 inches. While dogwoods is not necessarily required for dogwoods in natural areas, dogwoods in home landscapes are best treated with general fertilizer in late spring and early summer after the leaves have completed expansion.
Never use fertilizers containing weed killer and always water after fertilization. Sometimes a soil test can be beneficial in determining the fertility of the soil in question. Additionally pruning can be an extremely useful practice for your dogwoods. Pruning dead and damaged branches helps prevent and limit the spread of diseases, while also helping the structual form and habit of the tree. Pruning is most advisable during the winter months when the tree is dormant. All pruning cuts should be made with clean sharp tools and in the proper order diagramed. Insulating roots from heat and cold, helping appearance, reducing erosion, and aiding in water conservation, mulching can be extremely beneficial to dogwoods. Much should be 4 to 6 inches deep underneath the limb spread. By mulching, dogwood owners also limit any mowing or weed trimming necessary beneath the dogwood. This alone can aid in reducing incidents of mechanical damage.
Mechanical and Herbicidal Injury
One of the most common types of mechanical injury to dogwoods is impact from lawnmowers. Injuries of this type can invite basal stemrot disease and insect borer infestation. Similarly, string trimmer impact can sometimes bruise or girdle the cambium (the vital layer of tissue under the bark).
Damage of this nature is not easily detectable until the tree suddenly declines and leaves begin to drop. Sometimes the placement of a permanent barrier around the trunk can help in avoiding these types of injury.
Dogwoods and other ornamental trees and shrubs are very susceptible to herbicides. Signs of herbicidal damage can be observed in distorted yellowing of leaves and browing and leaf scorch.
In all cases, herbicidal and mechanical injury places unnecessary stress on dogwood trees which typically results in reduced growth and sometimes death. Properly identifying the problem is always the best method in addressing and diagnosing dogwood concerns.
Dogwood Anthracnose - Dogwood Anthracnose causes leaf spots, stem cankers, and kills shoots. Caused by the fungus, Discula destructiva, if not addressed promptly and properly, infected trees may quickly decline. Initial symptoms are medium-large, purple bordered leaf spots and scorched tan blotches that may enlarge and kill the entire leaf. Blighted leaves will often cling to branches and stems after normal leaf drop in the fall, providing a dead giveaway. Trunk sprouts occur during the latter stages of the disease development. The fungus will infect twigs and tends to grow down a limb and infect the main trunk of the tree. Cankers are also know to form and can be detected when the bark is peeled back. Cankers have the appearance of distinct margins tissue surrounded by healthy cambium tissue.
Because dogwood trees that are adjacent ot natural stands of other dogwoods are at more risk to catch Dogwood Anthracnose, special attention should be given to eliminating dead wood via pruning and destroying any diseased wood that was pruned immediately to avoid potential sources of fungus in the area. Fungicides registered for anthracnose on dogwoods can be used to treat incidents.
Spot Anthracnose - Spot Anthracnose is caused by the fungus, Elsinoe corni. This fungus can persist from year to year in infected twigs, fruit, and other tissues. With this kind of anthracnose, the flower bracts are usually the first to become infected. Occuring on leaves, young shoots, fruit, and flower bracts, the initial symptoms for Spot Anthracnose are small, reddish-purple leaf spots that are circular or elongated on the first bracts and leaves of spring.
The spots are typically small with yellowish centers and brown or black margins. As infection spreads, the spots merge to form larger leaf spots.
Severely infected flower bracts will typically fall from the plant prematurely. Disease will then spread to the foliage and the leaf spot centers will eventually drop out. Young shoots and berries may develop scabby, elongated lesions that have a distinct purple appearance along the borders. If the weather is dry during or before the early spring flowering period, very little disease can be observed. Infections typically vary from year to year depending the atmospheric moisture available. While fungicide sprays can be useful for Spot Anthracnose, most cases of the disease do not cause significant damage and sprays are not usually needed. If fungicide is available, apply to bud swell and bloom for treatment.
Septoria Leaf Spot - Some symptoms of Septoria Leaf Spot include medium uniform, purplish spots, or lesions on the leaves. While the lesions do not retain the deep, purple border, the centers of these spots become grayish in color. This disease typically occurs near the end of summer on stressed or weakened trees. Generally more of a problem when wet conditions and high humidity persist.
Botrytis Petal Blight - Botrytis Petal Blight is caused by the fungus, Botrytis cinerea. This fungus affects foliage and green shoots as well as petals. Generally observed during wet spring weather, this disease is typically common following winter injuries since it is produced on diseased tissue during periods of high humidity. Spores of the fungus are spread by wind and water.
Usually, this disease does not cause significant damage. Fungicides are not typically needed.
Trunk Canker - Basal Trunk Canker is caused by the fungus, Phytophthora cactorum. This disease is a major problem for urban dogwoods and is generally associated with poorly-drained areas. Injuries caused by mechanical injury and insects are ideal sites for the fungus to infect.
Leaves of infected trees are stunted and generally turn red prematurely. More seriously, the basal portion of the trunk sinks, causing constriction and decay and the bark of the trunk eventually peels away, exposing the wood tissue. Some bleeding from the canker can occur, and sprouting may occur below this damaged area.
Powdery Mildew - Dogwood Powdery Mildew is mainly caused by the fungus, Erysiphe pulchra. A related, but different fungus, Phyllactinia guttata, can also be found on diseased dogwoods in Kentucky. Often immediately identified by its milky white coating,
Powdery Mildew is rarely lethal and is typically caused by high humidity instead of excessively wet conditions. Powdery Mildew is an obligate parasite, preferring fresh, succulent plant growth. Dogwood Powdery Mildew occurs in open landscapes as well as in heavily shaded areas. Affected foliage of this disease typically will turn yellow and develop brown spots. Some leaves may appear scorched as a result. The best prevention and treatment of Dogwood Powdery Mildew is to purchase cultivars more resistant to the disease (or a C. florida x C. kousa hybrid dogwood) and acquire fungicides for application when necessary.
While insect damage to dogwoods is very common and in most cases considered negligible, some damage, when coupled with other sources of stress on the dogwood, can become serious and present a potentially life-threatening danger to the tree.
Dogwood Twig Borer and Clubgall Midge - Both of these insects can cause tip dieback and damage that usually
appears as withering leaves on branch tips during the growing season. Minor infestations are best treated by pruning the affected area and destroying the dead twigs and branches yearly.
Dogwood Borer - The dogwood borer is a common insect pest on established dogwoods. The larvae of the borer enter the tree through openings in the bark and feed on the cambium layer of the tree. Most attacks occur around basal wounds caused by mechanical injury. The best prevention is to protect the trees from wounds; however, the borers can be treated with any common systemic insecticides available on the market.