Mother Earth Living

Natural Garden Pest Control

Common herb problems and what to do about them
By Betsy Strauch
June/July 1998
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This parsley worm, the caterpillar of the black swallowtail butterfly, is at home here on a dill plant. It’s a beauty that most herb gardeners welcome, and it usually does only minor damage.
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Summer is the season for bumper crops of beautiful, healthy herbs, time to show off your knee-high basils or waist-high lavenders to wide-eyed visitors. But sometimes the reality in our gardens doesn’t match dreams nurtured by the alluring copy of mail-order catalogs and the glossy photos in gardening magazines. Are spindly, undersized plants withering in the sun in hard, cracked soil? Are your herbs leaning toward the sun from the shade of tall trees or buildings? Are the leaves you were expecting to harvest yellowish, curled, holey, spotty, or covered with white fur? Disappointing results like these can take all the fun out of gardening. Fortunately, there’s lots you can do to prevent and solve these and other common problems that can occur in an herb garden.

Satisfy your Plants

Many problems in growing herbs can be avoided simply by giving plants what they need, which starts with thorough soil preparation and careful siting. Some herbs, such as thyme, rosemary, lavender, and oregano, need a sunny exposure and soil that drains readily. Others, including basil, parsley, and dill, also grow best in full sun but prefer rich loam. Wild ginger, bloodroot, and other woodland natives need shade and soil that stays moist. To plant lavender in moist shade or wild ginger in dry sun is to kill it or at least stunt its growth.

Any soil, whether clay, sand, or loam, needs to be loosened to enable oxygen to get to the plants’ roots. Adding organic matter such as compost or well-rotted manure helps sandy soil retain water longer and opens up clay so that it drains better. You probably can’t overdo the compost; a wheelbarrow load of it thins to almost nothing as you rake it over the bed. I also scratch in 5-10-5 garden fertilizer at the rate recommended for vegetables; get a soil test if you’re not sure how fertile your soil is. (One clue to whether your soil will grow herbs is weed growth: if the weeds look sickly, it’s unlikely that herbs will do any better.)

• If heavy clay soil drains poorly even after amending it, consider planting herbs in raised beds (filled with ­better-draining soil) or in containers.

• Inspect plants at the nursery before you buy and reject any rootbound, leggy, or buggy ones.

• Whether you’ve purchased seed­lings or raised them yourself, harden them off before transplanting outside.

• Wait to set out basils and other ­tender herbs until air and soil temperatures are above 60°F.

• Find out the mature size of each kind of plant and leave enough room between the plants for growth. If the garden looks bare at first, you can fill spaces temporarily with annuals or other plants, but be prepared to remove them when the garden gets crowded.

• Mulch to retain moisture in the soil and keep roots cool. Mulching also keeps down weeds, which compete with herbs for water and nutrients.

Looking for Trouble

Even planting your herbs at the right time in the right places in the right soil won’t prevent problems ­entirely. Although there’s not too much you can do if a tornado or plague of locusts strikes, there’s plenty you can do to combat lesser problems. Always look for the least toxic solution to a plant problem.

First, familiarize yourself with what’s normal—something like self-examination for breast or skin cancer. Walk among your herbs every day. Check leaf color, shape, and texture, and whether plants appear to be increasing in size. Frequent monitoring will help you spot and deal with problems as soon as they arise and before they do a lot of damage.

If you notice something amiss, determine whether it affects just one plant (or part of a plant), most or all of the plants of the same type, or all the plants in that bed or part of the garden. Also try to evaluate the severity of the problem: if bugs show up but soon go away or seem to be causing little damage, you may not need to do anything.

If the foliage appears chewed or discolored but you can’t see what might have done the chewing or discoloring, shake a branch over a piece of white paper and see what falls on it. Are any of those specks moving? You may need a hand lens or magnifying glass to see spider mites or other minute critters. Identifying what you’ve got will enable you to deal with the problem effectively. Ask a local extension agent for help. I often turn to my 1966 copy of the Golden Nature Guide Insect Pests for help in pest identification (though its information on controls is thirty years out of date).

Here are some common problems of herbs and suggestions for dealing with them.

Pests

Aphids are tiny sapsucking insects that love the new, tender growth of angelica, calendulas, caraway, lovage, mints, nasturtiums, oregano, peppers, sage, southernwood, and roses. New leaves may be curled and sticky with their excreted honeydew. Small numbers are easily controlled by squashing. Spray insecticidal soap on large infestations, but don’t spray if you see ladybird beetle larvae on the same plants: they eat aphids.

Iris borers are the larvae of a moth that lays its eggs in the fall on old leaves and flower stalks of orris (Iris germanica var. florentina) and other irises that grow from rhizomes. In late April or early May, the emerging pink caterpillars munch on the leaves and then burrow into the rhizomes. If you notice ragged, wilting, or rotting leaves, dig up the plants, then cut away and destroy chewed or rotten parts of the rhizomes—bacterial soft rot also may afflict borer-damaged rhizomes. Replant the sound parts. In the fall, remove any shriveled leaves or other debris to prevent further infestation. Some authorities recommend pinching nibbled leaves in spring to crush the borers inside. Recent trials at the University of Maryland indicate that microscopic nem­atodes that infect borers with a deadly bacterium are an effective biological control (although if you have only a few plants, mechanical control is easy and presents an opportunity to reset your irises in amended soil at the same time).

Japanese beetles are a notorious scourge of roses in the eastern United States, but they also attack basil, echinacea, foxglove, and many other herbs. The larvae (grubs) feed on the roots of lawn grasses. The glossy bronze-and-green adults emerge in early summer. They feed during the day, preferring plants in full sun. Hand-picking or brushing the beetles into a container of soapy water is easy early in the morning or at dusk, when they are sluggish. Look for them at these times on the tops of taller plants such as asparagus, pole beans, lavatera, currants, and raspberries. Trapping may work if everyone in the neighborhood does it, but if you’re the only one with traps, you may attract your neighbors’ beetles to your yard. Treating the soil with milky spore disease to kill the grubs is fairly effective where soil temperatures are warm in early fall but less so in northern states.

Parsley worm is the handsomely striped caterpillar of the black swallowtail butterfly. You’ll find it on parsley, of course, but also on dill, rue, and other members of the carrot and citrus families. The adults obtain nectar from milkweeds, thistles, phlox, and purple loosestrife. Plant enough larval host plants to share with the caterpillars if you want to have black swallowtails flitting through your garden.

On the other hand, if a flock of caterpillars (of whatever kind) is eating up your entire crop of oregano, for example, you may just want to get rid of them. If hand-picking is out of the ­question, you can spray or dust with Bacillus thuringiensis (Bt), an effective biological control that paralyzes caterpillars so that they die from starvation.

Snails and slugs can nibble your favorite herbs down to nubbins. Their favorites include basil, calendulas, marigolds, sage, sorrel, and violets. They are most active in wet weather and at night. Hand-picking (dropping them into soapy water) is effective, or you can snip slugs in two with pruning shears. Putting out boards or melon rinds for snails and slugs to shelter beneath has little effect where soils stay moist. Baiting with tuna cans filled with stale beer or a yeast solution is more trouble than it’s worth, in my opinion. Copper strips, which are toxic to the mollusks, may be installed around a bed but won’t drive out any of the animals that are already in residence. A ring of sawdust is ­reputed to make an effective barrier and costs little or nothing.

Spider mites are minute pests related to spiders and ticks that suck the sap of many herbs including angel­ica, lemon verbena, mints, oregano, roses, rosemary, sage, thyme, and violets. Look for stippled leaves. Heavily infested leaves may have webbing on the underside. Spider mites are especially active in hot weather. Keeping plants well watered washes off mites and webbing while replenishing lost fluids. You can also spray plants with insecticidal soap (even though mites aren’t insects) or horticultural oil (for nonwoolly foliage only).

Spittlebugs are small, winged insects whose nymphs, or young, surround themselves with globs of foam for protection as they suck the juices from the foliage of herbs such as southernwood, lavender, rosemary, oregano, and fennel. If there are only a few, you can ignore them; however, they’re easily routed with a hard spray of plain water, or you can crush them by hand (you might want to wear rubber gloves).

Whiteflies are tiny white mothlike insects that are related to aphids; both adults and nymphs suck the juices from lemon verbena, nasturtiums, rosemary, and scented pelargoniums. The adults fly up when the plants are disturbed. Small numbers of whiteflies probably aren’t worth treating; use insecticidal soap on more serious infestations.

Diseases

“Melting out” describes various foliar diseases that flare up in hot, humid weather and quickly turn healthy plants to black mush. Herbs affected include sweet woodruff, southernwood, and thymes. Woolly-leaved herbs such as lamb’s-ears, artemisias, and yarrows are especially vulnerable. Cut back and ­discard the dead stalks; the plants may recover. Next season, increase air circulation by spacing plants generously and mulch with light-colored gravel or sand instead of moisture-holding organic material. If your summers are relentlessly hot and humid, choose other herbs. How about gotu kola?

Powdery mildews are fungi that whiten leaves of bee balm, calendulas, tarragon, yarrows, and many other plants in late summer during periods of warm days and cool nights. Fortunately, they are more unsightly than harmful. Cut back and destroy mildewy stalks of herbaceous plants. Next season before mildew appears, you may wish to spray with a fungicidal soap (fungicides prevent mildew but don’t cure existing cases). Insecticidal soaps, baking soda/horticultural oil solutions, and garlic extracts have been effective in deterring powdery mildews on other kinds of plants. Look for mildew-resistant cultivars such as the bee balms ‘Mahogany’ and ‘Marshall’s Delight’.

Root rots are fungal diseases that afflict plants that don’t tolerate constantly wet soil, such as clary sage, lavender, mullein, oregano, rosemary, tarragon, and thyme. Improve drainage with soil amendments and/or raised beds, and don’t overwater. Use a sand or light-colored gravel mulch and thin out stalks to increase air circulation. Thoroughly clean up plant debris in the fall.

Rusts are fungal diseases that produce rust-colored or black pustules on the leaves and stems of plants including bee balm, germander, mints, roses, and yarrow. Affected plants may be stunted or killed. Rust on mint is a big problem for commercial growers. Rusts thrive on wet foliage, so avoid overhead watering or water only early in the day so that the foliage will dry before nightfall. Pick off and destroy affected leaves of lightly infected plants; remove heavily infected plants. Thoroughly clean up plant debris in the fall.

Other problems

Sprawling of herbs such as Silver Mound artemisia, oreganos, or catmints can result from too much fertilizer and/or heavy rainfall. You can cut back stems severely or prop them up with wire or sticks. Go easy on the fertilizer.

Leaning of herbs planted in the shade is a sign that they need more sun. Move them to a sunnier spot.

Underwatering can stunt or kill; moisture-loving plants such as gotu kola are especially susceptible.


Betsy and Joe Strauch tend herb, vegetable, and flower gardens in Lenox, Massachusetts. Betsy is an assistant editor of The Herb Companion, and Joe is a garden ­photographer.

Further Reading

Buchanan, Rita. Taylor’s Guide to Herbs. Boston: Houghton Mifflin, 1995.

Chase, A. R., Margery Daughtrey, and Gary W. Simone. Diseases of Annuals and Perennials: Identification and Control. Batavia, Illinois: Ball, 1995.

Cooperative Extension Service Office publications on garden pests and diseases.

Ellis, Barbara W., and Fern Marshall Bradley, editors. The Organic Gardener’s Handbook of Natural Insect and Disease Control. Emmaus, Pennsylvania: Rodale Press, 1996.

Kowalchik, Claire, and William H. Hylton, editors. Rodale’s Illustrated Encyclopedia of Herbs. Emmaus, Pennsylvania: Rodale Press, 1987.

Olkowski, William, Sheila Daar, and Helga Olkowski. Common-Sense Pest Control. Newtown, Connecticut: Taunton Press, 1991.

Smith, Cheryl, ed. The Ortho Home Gardener’s Problem Solver. San Ramon, California: Ortho Books, 1993.


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