Natural Garden Pest Control

Common herb problems and what to do about them

| June/July 1998

  • Spider mites leave their mark on oregano leaves.
    Photography by J. G. Strauch, Jr.
  • Most bee balms are susceptible to powdery mildew in late summer.
  • These whiteflies are feasting on an osteospermum.
  • 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.
  • Meet the spittlebug. Ignore it or hose it off.
  • This spearmint has rust, an ugly fungal disease.
  • Oregano has a tendency to open up in the center. Stake or cut the stems—your choice.
  • This sweet woodruff is turning to mush. Foliar diseases plague many herbs in hot, humid weather.
  • This gotu kola is saying, “Water me! Water me!”

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.

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