Dogwood trees in the landscape

Published 1:18 pm Monday, July 2, 2018


Dogwoods are popular ornamental trees that are found in many garden landscapes, but they are very sensitive to their growing environment. The flowering dogwood (Cornus florida) is native to the eastern United States where it is the state tree for both North Carolina and Virginia.

When undisturbed in the shade provided by the canopies of larger trees, dogwoods thrive and can grow to be 40 feet tall and 80 years old. Dogwoods do best when grown in moist, well-drained, fertile acidic soil (pH of 5.5 to 6.0) that is high in organic matter. In addition, dogwoods prefer being grown in partial shade on well-mulched, undisturbed ground and not subjected to the heat from afternoon sun. Since dogwoods are shallow-rooted and do not tolerate dry and hot conditions, the soil must be kept moist at all times. When grown outside of this ideal environment, dogwoods become readily stressed and can easily succumb to a host of diseases and insects. For protection, timely applications of suitable pesticides can be made according to the labels. Environmental stress also causes dogwood decline, which results in poorer growth and leads to shorter tree life.

The most serious disease of dogwoods is a fungus called dogwood anthracnose, which has devastated flowering dogwoods in forests for the past 30 years. Starting with the lower branches and progressing up the tree, the infected leaves develop small tan spots with purple edges that merge into large tan blotches. The disease progresses to dieback of twigs and then the lower branches with the formation of cankers. Infection occurs mostly during cool, moist conditions in the spring and fall. After symptoms first appear, older trees die within three to four years, while younger tree usually die the same year.

The disease is most prevalent in locations having altitudes over 2,000 feet. At lower altitudes, the most significant infections have occurred to trees growing in cooler, wet and shaded environments. When grown in open sunny sites, infected dogwoods often become disfigured but still live on. Where the disease is known to be present, protective fungicides, such as chlorothalonil (Daconil) can be applied every seven to 14 days when conditions favor the disease. In addition, there are a number of disease-resistant dogwood cultivars, particularly the oriental dogwoods (Cornus kousa).

Spot anthracnose is another fungal disease that causes similar spotting on the leaves and flower bracts, often occurring during cool, wet conditions in the spring. Usually spot anthracnose does not result in significant damage, but it can weaken the tree. Other fungal leaf diseases include Cercospora and Septoria leaf spot, which appear from midsummer to early fall. Timely applications of a suitable fungicide can provide protection.

To the untrained eye, leaf diseases are difficult to distinguish from one another. Dogwoods are also susceptible to powdery mildew, which is a fungal disease that appears as a white to light-gray powder on the leaves. It occurs during the spring and fall months under cool and wet conditions in conjunction with poor air circulation. The effects are usually not life-threatening, but the non-resistant varieties of dogwoods can be protected by timely applications of a suitable fungicide, such as myclobutanil (Immunox).

When grown in poorly drained soil, dogwoods are also prone to root rot (and shoot blight) caused by soil-borne fungi phytophthora and armillaria. Once contracted, root rot is difficult if not impossible to cure.

The most damaging insect to dogwoods is the dogwood borer, which also attacks oak, hickory, pecan, willow and apple trees. Small moths that have somewhat clear wings lay eggs on the bark usually in the upper parts of the trees during May and August. After about nine days, the larvae enter the bark through any broken, calloused or canker areas. The borers feed on the inner bark, which causes girdling of the tree and eventual death of limbs and the trunk. About a year later, the borers emerge as moths. In the egg and larvae stages during late April and mid-July, borers can be control by spraying the trunk and lower limbs with a suitable insecticide (such as permethrin or bifenthrin).

The kousa dogwood (native to China, Japan, and Korea) will grow to 30 feet tall. Most varieties have better resistance to both dogwood and spot anthracnose, powdery mildew, and dogwood borers. Kousa varieties, however, still require fertile, moist, well-drained soil and prefer shade but will tolerate some afternoon sun.

How successful dogwoods and other plants will prosper largely depends upon doing the homework to choose the right ones to plant in the right places at the right times. In most situations, the decline of ornamental trees and shrubs stems largely from cultural practices such as plant injury from weed eaters and mowers. Lastly, replacing problem trees and shrubs with other, more suitable ones is often more cost effective than continuing to bear the cost of applying pesticides.

This information is given courtesy of the Dan Bergbauer, master gardener, and volunteer Extension master gardeners under the direction of the Beaufort County Cooperative Extension. Volunteers are available through the hot line to answer your questions concerning lawns, vegetables, trees, ornamentals, fruits, plant problems and diseases, pests, soil and many other horticulture issues relating to gardening in North Carolina. The hot line telephone number is 252-946-0111. You can leave a voice message at that number or email the master gardeners your questions at and a volunteer will return your call.


Gardening Calendar July

Avoid gardening between the hours of 10 a.m. and 4 p.m. to minimize the stress to the plants and to the gardener.

Lawn Care
Fertilize warm-season grasses like Bermuda, Zoysia and St. Augustine, if you haven’t already done so.
A 1/3rd of the growth should be removed when mowing warm season grasses.
Try to change direction when mowing your lawn. This will help strengthen the roots system and expose different sides of the plant to sunlight.

Continue side-dressing your garden vegetables.
July is the last month to fertilize landscape plants.
Now is a good time to take soil samples from your lawn. Soil boxes can be picked up at the County Extension office.

Vegetable to be planted in July: brussel sprouts, collards, beans, carrots, tomatoes and pumpkins.
Start broccoli, cabbage and cauliflower plants in peat pots to be transplanted in mid-August.
Now is a good time to repot overgrown houseplants.

“Bleeder” trees like maple, dogwood, birch and elm can be pruned this month.
Outgrown hedges can be pruned.
Pinch off garden mums till mid-July.
Narrowleaf evergreens like junipers and arborvitaes can be pruned.
Fruiting canes of raspberry and blackberry can be cut down to ground level after the harvest.
Remove faded flowers on flowering perennials to encourage a second flowering.
The dieback on hybrid rhododendrons, azaleas and blueberries can be pruned out
Lastly, don’t forget to pinch your chrysanthemums to encourage branching.

Watch shrubs for the following insects: bag worms, leaf miners, aphids, spider mites and lace bugs.
Japanese beetles if needed.
Use recommended herbicide to control poison ivy and honeysuckle if desired.
Start fungicide treatment on tomatoes, which show signs of blight.
Continue with rose spraying program.
Continue fungicide program for fruit trees and bunch grapes.
Spray the following vegetables if insects are observed: cucumber (cucumber beetle), squash (aphids), tomato and eggplant (flea beetle).
(Pesticides should be used sparingly! Use only when needed and always follow the label)

Other Activities
Prune any branches damaged by the recent storms.
Fruit trees that bore no fruit like peach, plum and apple can be pruned as if they were dormant. This will allow more sunlight in and prepare the plant for the next year.

Tips on Shopping at the Farmers Markets
Fresh green beans should “stick” to a T-shirt.
The stem ends of fresh tomato should have a white color.
Smell may not always be a clue to a cantaloupe’s freshness. Older varieties of cantaloupe have a great smell however; newer varieties may not have the same smell but taste just as good. No cantaloupe should smell bad. A bad smell is a sign of bacteria and spoilage.