Invasive Plant, Pest and Disease Awareness Month sheds light on problem

When a plant is listed as invasive, how far does it spread? How far apart should I plant bamboo for a privacy screen?

How far? "Invasive" doesn't mean garden-variety nuisance — it means a foreign invader spreading aggressively until it has destroyed environment, economy, even human health. Sometimes, it is unstoppable. April is Invasive Plant, Pest and Disease Awareness Month, a time to consider what we lose when invasive species get in the United States. For example, Emerald ash borer has killed tens of millions of ash trees and is now in Maryland. So far, no ash tree can resist it. The giant African snail was brought to the U.S. in 2011; it eats 500 different plants and carries infectious meningitis. Do you enjoy orange juice? Citrus greening disease is threatening the orange-growing industry. Rock snot — a type of algae — is smothering Maryland streams. These are only a few invasive species in the U.S. Out-of-control bamboo is one of our most frequent problem calls at the University of Maryland Home and Garden Information Center. Contact us and we can suggest alternatives.

My redbud branch broke in an ice storm, leaving a ragged stump. Should I fill it with cement? Spray with that black tar-like gunk made for plant wounds?

Water will invariably get behind cement or plug materials and rot the tree. As for the "black gunk," research shows that wound dressings actually slow down the healing process. A tree can heal a jagged wound when necessary, but your best bet is to prune the cut back as smoothly as possible. Most important when removing branches is to leave the branch collar — the raised area where the branch attaches to the trunk or larger limb. The collar contains chemicals that help cuts "heal." At the same time, don't leave a stub protruding out from the collar. Stubs will rot and provide an entrance for disease and pests. For simple tips and diagrams, see our website's publication, "HG 84: Pruning Ornamental Plants."

University of Maryland Extension's Home and Garden Information Center offers free gardening and pest information. Call 800-342-2507 or send a question to its website at

Plant of the week

Lenten rose, hellebore

Helleborus orientalis

Hellebores have flowers for an incredibly long time — late winter to May — with petals in smoky purple, speckled mauve, white and pastel green. Even in summer, the tough blooms fade but persist. Thankfully, deer usually snub the foliage and flowers. The leathery, heavily serrated leaves emerge bright green in spring, then darken. After winter, allow old foliage to decompose where it collapses around the base of the plant like a mulch, or prune it away to prevent disease. Hellebores self-seed, and enlarging clumps can be divided. Plant in moist shade or part shade, ideally on a slope, so you can look up at the nodding blooms.

—Ellen Nibali

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