After more than 30 years, I figured there wasn't much more for me to learn about growing roses. I pruned once a year, fertilized once a month, watered once a week, and dead-headed flowers once in a while. But most of the time I spent in my rose garden, I was spraying to control insects and diseases.
About 10 years ago, I began to question all my pesticide spraying. Was it for beautiful blooms? Perfect foliage? And what were the health consequences? Was there an easier, less toxic way to control pests and diseases in the rose garden?
While all this research is ongoing, I can share with you what I've learned.
The most common diseases here in north-central Texas are black spot and powdery mildew. Typically, roses with yellow flowers or yellow-flowered ancestors are the most disease-prone. Among the antique roses, Bourbons, Kordes, and Hybrid Perpetuals are the most prone to black spot.
Aphids, thrips, June (or May) beetles, cucumber beetles, and spider mites are the most common insect and mite pests in my garden. This differs somewhat from troublesome pests in other regions. (See below for a more complete listing of rose pests and their controls.) Again, some generalizations: Light-colored roses -- whites, yellows, and pinks, both moderns and antiques -- are the most susceptible. As a rule, the modern classes, with their high, pointed centers and tight bloom structures, are generally infested sooner and are more difficult to cure.
I like to grow many kinds of roses including some of the more pest-prone kinds. But there are many roses, modern and antique, that are rarely bothered by rose pests of any kind. Seek out these kinds of roses if you want roses but don't want to do any spraying or fussing. Some of the most trouble-free include 'Bonica' (pink), 'Carefree Delight' (pink); 'Carefree Wonder' (pink), 'Cecile Brunner' (pink), 'Livin' Easy' (orange blend); 'Martha Gonzales' (red), 'Old Blush' (pink), and 'Vanity' (pink).
Rose gardeners today have at their disposal some exciting new materials, new techniques, and even some old materials used in new ways. I have tried most of these least toxic controls and can report here how they fared.
1. Anti-transpirants, such as Wilt Pruf and Cloud Cover, are generally believed to work as fungus controllers by coating the leaves with waxes, plastic polymers, or silicones, thus preventing fungal spores from entering the pores in the leaves. In my garden, they provided reasonable control but only reduced the spread of disease; they did not eliminate it. Apply just as you would an anti-transpirant; follow label directions.
2. Baking soda combined with horticultural oil has proven to be the most effective method of eliminating powdery mildew in my garden, and after only one or two applications. This fungicide also or eliminates or reduces black spot to acceptable levels on resistant rose varieties.
For years, the most effective formulation for me is one rounded tablespoon (approximately four teaspoons) of baking soda (sodium bicarbonate) mixed with one tablespoon of horticultural oil per gallon of water, which is sprayed on a weekly basis or after a heavy rain. Baking soda can burn leaves: Apply in early morning and not at all during hottest weather.
Dr. Janell Johnk, Extension Plant Pathologist at Texas A&M University in Dallas, helped develop the use of potassium bicarbonate for powdery mildew control. Look for potassium bicarbonate under the brand names of Remedy (from Bonide Co.), and Kaligreen (from Toagosei Co.). Applied at the same rate, it promises somewhat greater activity against the powdery mildew fungus, but it will burn leaves during hot weather at least as readily as sodium bicarbonate.
3. Sulfur-based fungicides have long been the organic gardener's weapon of choice for battling fungal diseases. In my tests, they provided reasonable control but showed residues on the leaves and petals and cannot be used when temperatures exceed 90°F. Plus, sulfur sprays are difficult to apply -- they clog my sprayer regularly.
4. Whole neem oil. Unlike neem oil extracts used to control insects (see below), whole neem oil controls black spot, powdery mildew, and rust as well as many insects and mites. It's also less expensive than neem oil extracts.
Whole neem oil is 90 percent "clarified hydrophobic extracts of neem seeds." (Neem oil comes from the tropical neem tree, Azadirachta indica.) Translanted, this means the oil is cleaned up but is not refined and that it doesn't mix very well with water. This latter feature is important: Only mix whole neem oil into water that's warm or at least room temperature.
Neem oil is different from neem oil extracts, which have been available for a few years, and is currently available from only one company. The product name is Rose Defense and it is available from the Green Light Co. It costs $15 per pint and you need 2 tablespoons per gallon of water.
Insect Pest Sprays
5. Water. Nothing fancy here, just plain old water. Keeping plants clean helps prevent problems with spider mites before they start, and a strong spray of water can knock aphids, spider mites, and other pests off plants so you don't have to spray. There's even research suggesting that warm water is mildly fungicidal! So don't forget to include water in your least toxic program.
6. Bt (Bacillus thuringiensis). This microbial insecticide revolutionized pest control methods in the 1970s when it first became widely available to home gardeners. It kills only moth and butterfly larvae and is harmless to most other insects and animals. However, don't spray throughout the garden or excessively. It won't discriminate between pest caterpillars and those of desirable moths and butterflies. Many types are available from various manufacturers.
7. Horticultural oil. A more highly refined version of traditional "dormant" oils applied to leafless trees and shrubs in winter, horticultural or "summer" oils control a wide variety of rose pests, including rose scale and whitefly. These oils also control soft-bodied pests, such as aphids (and their eggs) and spider mites. Don't use horticultural oils when you expect temperatures to rise above 90°F.
8. Insecticidal soaps. These specially formulated soaps are a key element in any least toxic pest-control strategy. They are effective against a wide range of mite and insect pests, particularly soft-bodied insects such as aphids, immature scale, leafhoppers, mites, thrips, and whiteflies. Soaps are essentially nontoxic to humans and wildlife, but they can damage some plants, especially ones with dull leaf surfaces or many leaf hairs. Luckily, few roses fall into those categories. Also, do not spray soaps onto plants that are water-stressed or during hot, windy, or humid weather.
While soaps have minimal impact on beneficial insects, I don't spray the entire garden because you risk killing any predatory insects. Instead, I spray each flower bud just as it begins to show color. The effect of this minimal spray lasts about three days -- long enough to discourage adult thrips from laying eggs.
The best time to apply insecticidal soaps is early morning or late afternoon. Cost and application rates vary according to the product.
9. Neem oil extracts, such as Safer's Bioneem, contain azadirachtin-A, the substance believed to be the most insecticidal ingredient of neem oil. This product provides the most effective spray control against insect pests in my rose garden. And unlike many other pesticides, synthetic and organic, neem is not harmful to most beneficial insects.
Label instructions specify three applications at 7- to 10-day intervals. Eight ounces costs $10, and each application requires three ounces. Clearly, good timing is essential not only to pest management but also to most economical usage.
Biological Controls of Insect Pests
These are natural insect predators and parasites that benefit the gardener by controlling a specific pest. They harm neither plants nor people. Utilizing beneficials has proven to be the overall most effective way to eliminate insect and mite problems in my rose garden.
Because beneficial insects are living organisms, timing their release into your garden is critical. But once successful, re-releasing them is only necessary if pest infestations occur before the "bennies" appear in sufficient quantities to control their prey.
Here are my choices and their targeted pests:
10. Beneficial nematodes (Steinernema and Heterorhabditis) are microscopic wormlike creatures that parasitize soil-borne larvae of pest insects. Their target prey in my garden are June beetle larvae (called white grubs), cucumber beetle larvae (called Southern corn rootworm), and the emerging larvae of the rose stem borer (the larval form of a beetle). Apply only once soils have warmed in spring.
11. Green lacewing (Chrysoperla carnea) larvae. Release when temperatures warm in late spring. The beautiful green-bodied, golden-eyed adults eat aphid honeydew, mate, and have more offspring. But the alligator-looking larvae take on all comers. They consume aphids, spider mites, scales, thrips, and the eggs of many more pests.
12. Lady beetles (Hippodamia convergens). I buy two "tubs" of 1,500 to 3,000 lady beetles each in March -- as soon as I see aphids in the garden. One is for the front yard and one is for the backyard. Each tub costs about $7. This one application keeps aphids in check for the rest of the year.
13. Mini-wasps (Trichogramma). These gnat-sized wasps attack most caterpillars of moth and butterfly species. The advantage to using these biocontrols is that they attack the eggs of their prey, thus preventing the pest larvae from even hatching. Release once temperatures will remain above 72°F.
14. Predatory mites (Phytoseiulus persimilis). Release when temperatures rise above freezing in the spring. These mites reproduce twice as fast as the two-spotted spider mite, their primary prey. This makes them extremely effective in controlling pest mites.
Now there is a practical and effective least toxic remedy for all the major rose pests. The numbers after each pest below correspond to the 14 remedies listed in the article.
Mark Whitelaw was a member of the executive board of the Fort Worth Rose Society. He passed away in 2000, but his copious writings teaches us still.
Photography by Michael MacCaskey (top) and the USDA (bottom)
Article published on June 23, 2008.