Grasses' Pests And Diseases

Grasses are still among the most pest-free and disease-free of all garden ornamentals. When properly selected and placed in the garden, grasses are generally disease-and insect-free. Though some grasses are more prone to problems than others, most are easy to grow and undemanding.

Miscanthus mealybug

Miscanthus mealybug is a serious pest that fortunately appears to be restricted to the genus Miscanthus. The mealybug, Miscanthicoccus miscanthi, is an Asian native. The mealybug is not capable of moving long distances unassisted; in fact, it probably cannot spread more than a few feet on its own. It has been dispersed unwittingly but entirely by the sale and exchange of infested plants. The presence of this insidious pest is not usually noticed until the population on an individual plant builds to high levels and superficial symptoms become readily apparent.

Up to 3/16 in (4 mm) long, the mealybug lives in the tight space between the stem and the enclosing leaf sheath. Colonies are usually established first toward the base (crown) of the plant and spread upward as their numbers increase. The mealybugs are difficult to see until they reach mature adult size. The best way to confirm their presence is to pull a lower leaf sheath away from the stem of an infested plant. There, the white-powdery wax and syrupy honeydew that is produced by and obscures the individual mealybugs can be seen.

A general stunting of growth and an uncharacteristic twisting in the flower heads are usually the first superficial symptoms. The stem and sheath tissue often turn dark red in areas where the mealybugs are feeding, especially in late season. Severely infested plants are not killed, but are reduced to unsightly, misshapen masses with white powder covering the stems, especially in the lower portion. Affected plants often fail to flower at all, or the stalks of the inflorescences may be stunted, causing the flowers to open down among the foliage rather than gracefully above.

Snails and slugs

Snails and slugs can do far more damage to certain grasses than insects do. Most grasses are not affected by slugs and snails, and no treatments are necessary. Succulent-leaved grasses, subtropical grasses, and sedges are most susceptible to damage. A strip of copper edging or diatomaceous earth will deter slugs. Handpick at night, dropping slugs in soapy water, set out beer traps, or sprinkle with salt.

Gophers

For gardeners, gophers are the greatest menace of all to ornamental grasses. A single gopher can raise havoc with a grass planting. Gophers tend to eat the roots right at the crown of the plant, often shearing it off at ground level. In areas where gopher infestations are severe, use wire baskets to prevent gophers from reaching the crown. Place grasses in large baskets made of chicken wire or a smaller mesh. A 2- to 3-foot basket is ideal.

Rabbits

Rabbits are another major problem for grasses. Certain grasses are like dessert for rabbits. Often, rabbits do not bother grasses with wiry, fine-textured, or sharp leaves. If natural food is available, rabbits will usually not prove a problem. Use a fence to keep rabbits out of your garden.

Deer

Deer are rarely a pest of grasses. Sometimes they will lightly browse succulent new growth; however, once the grass has toughened, deer will leave it alone.
Deer will generally try any new plant in the garden once, if not twice. Usually they do not like sharp-edged leaves and so will leave most grasses alone. In areas where deer are a problem, stick to growing sharper-leaved grasses, and consider erecting a deer-proof fence.

Rust

Rust is a fungal disease that shows up on ornamental grasses as orange spots, which can spread to cover the leaves. Remove infected leaves as soon as you see them. Clean up the garden at the end of the season, removing and destroying rust-infected plant material. If your grasses were troubled by rust last season, prevent its return by making periodic applications of wettable sulfur, starting several weeks before the disease normally strikes. Water the ground, not the foliage, and space susceptible grasses far enough apart for good air circulation.