Roses are not the easiest plants to grow in the Washington, D.C. area.
Our climate is perfect for the development of black spot, a fungus that
can defoliate and weaken plants if not kept in check. Fortunately, antique,
heritage, and species roses are generally more resistant to diseases and
pests than hybrid tea roses. When control measures are needed, the Arboretum
practices Integrated Pest Management (IPM) to control pests and diseases.
The practice combines planting disease resistant varieties, promoting proper
cultural techniques, and careful monitoring of pests combined with spraying,
natural control methods, and tolerance of minor amounts of damage. To minimize
black spot problems and limit the amount of spraying you need to do in
your own rose garden, follow these tips:
Promote air circulation and light penetration. Prune trees and hedges
surrounding the rose garden low so that sunlight and breezes can quickly
dry the foliage after morning dew or rain. Keep companion plantings at
a distance to allow maximum exposure of the rose foliage to sunshine and
air.
Eliminate overhead irrigation. Water the soil with a soaker hose
and keep the leaves dry. Black spot spores need water on the leaf surface
to germinate.
Prune cankered canes. Cankered stems bear black, dead areas that
harbor the fungus over the winter. Prune them out, preserving more vigorous
healthy canes, to prevent infection of spring foliage.
Apply lime sulfur just before bud break in spring. This treatment
delays the onset of black spot and powdery mildew infections for several
weeks, even if weather is favorable for infection.
Apply a fungicide based on neem seed extract beginning when leaves are
fully expanded. Neem products are not as toxic as conventional fungicides
and have the added benefit of controlling many rose insect and mite pests.
They only work to prevent new infections, so spraying must be done on a
weekly basis as long as weather is humid or wet.
Apply conventional fungicides as a last resort. If the weather is
dry in early summer, and you have applied neem every week, you may not
have a black spot problem at all. But if you miss an application, or daily
rain and dew create ideal fungal infection and growth conditions, you may
need to use a conventional fungicide. Use a fungicide containing chlorthalonil,
thiophanate methyl, or propiconazole. It's best to rotate different fungicides,
never using the same one two times in a row, to prevent the fungus from
developing resistance to the fungicides.
Tolerate some damage, especially late in the season. Heritage and
antique roses generally bloom in late spring and bloom only sporadically
throughout the rest of the season. In July and August, with the flowers
gone, the plants attract little attention and some black spot can be tolerated
without permanent harm to the plants.
How should I prune my roses?
Prune your roses lightly in autumn, removing canes long enough to be
whipped by winter winds and those canes with signs of disease. Pruning
to remove remaining dead, diseased, and damaged canes is done in early
to mid-March just before growth starts. Species and climbing roses are
pruned by removing entire canes all the way to the ground to encourage
an open, vase-shaped habit. The rest of the roses get pruned to knee height
at an outward facing bud. You can do some light pruning to shape the plants
during the summer as needed.
Where can I get more information about roses?
Start with the American Rose Society located at www.ars.org
where you'll find a wealth of information about roses. If you
are looking for a specific rose variety, check our Plant
Sources Page for tips on finding a supplier.
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