Common Problems With Rosemary Plants

If you're interested in growing herbs, consider adding rosemary (Rosmarinus officianalis) to your list. Its delicate foliage is an attractive grayish-green and the plant fills the environment with its herbal aroma. Rosemary is generally easy to grow, but it might develop some problems if it's not given correct conditions or develops disease or pest problems. Catching these problems early can be essential to keep a rosemary plant in tip-top shape.

Rosemary grows outdoors year round in U.S. Department of Agriculture plant hardiness zones 8 through 10, but it also makes a good potted houseplant. Native to rocky and sandy Mediterranean regions, it requires excellent drainage to thrive. The plant only needs a modest amount of water and tolerates drought well once established. But like any plant, insufficient water can cause its foliage to wilt; if this happens, test the soil with your fingertip and, if it's dry several inches deep, water the plant well.

If you've caught the problem in time, the plant should revive and be undamaged. In future, water whenever the top few inches of soil feels dry to your fingertip.

A wilting rosemary plant can also indicate over-watering, a situation that tends to promote root rot. If the problem's not corrected, roots become slimy and soft, and stems wilt and eventually die back. Overly wet conditions can promote other fungal problems, such as powdery mildew, a disorder that causes fluffy white fungus to grow on stems and foliage; eventually, these plant parts dry up and die. In either case, trim away damaged parts of the plant, using shears to cut into healthy tissue below the damaged areas. Clean your blades with rubbing alcohol between cuts to prevent spreading disease.

If the plant's soil is wet and soggy, and some roots are affected, remove the plant from the soil, trim off damaged roots and replant the rosemary in fresh soil. You can also dust cut roots and stems with cinnamon, a natural fungicide, to help the plant recover. If all the plant's roots are affected, the problem probably can't be corrected and it's best to discard the plant.

A rosemary plant can also attract several types of pests. These include aphids, which are soft-bodied greenish pests, and spittlebugs which leave a white, frothy liquid on the plant. Whiteflies, which are tiny whitish flying insects, might also appear. You might also see evidence of spider mites, microscopic pests that leave visible webs covering young leaves and stems. All these pests suck plant juices, causing foliage to wilt, dry up and eventually drop from the plant.

Control a minor infestation by washing them off the plant with a strong stream of water. If an infestation becomes severe, spray the rosemary plant with insecticidal soap until it's dripping wet. Dilute the soap at a rate of 5 tablespoons per gallon of water, and repeat the spray every week or two, as needed.

Planting rosemary in the garden in a spot that gets full sun and has excellent drainage helps prevent problems with fungus. Rosemary can spread to reach a width of 2 to 4 feet, so space multiple plants at least 2 feet apart to ensure good air circulation, which also helps prevent fungal growth. If your soil contains clay and tends to drain slowly, add some coarse sand at planting to improve its drainage, or plant rosemary in a raised bed.

If you grow rosemary indoors as a houseplant ensure the container has a drainage hole, keep it well-spaced from other plants and always let the plant drain well after watering; never let its pot sit in a water-filled saucer.

Whether indoors or in the garden, check plants often for pest problems, because these are best controlled when caught early, before damage is severe.