Beware of Weedy Hitchhikers

Ellen C. Wells

Container-grown ornamentals are the backbone of the horticulture industry, but the Weed Science Society of America is advising vigilance to prevent the spread of weed seeds. Researchers with the USDA Agricultural Research Service recently investigated the problem of nonnative invasive plants and weeds in Alaska, and found some interesting conclusions.

“The team examined container-grown plants to see if they were a contributing factor,” said Lee Van Wychen, science policy director for the Weed Science Society of America. “The research showed significant numbers of nonnative weeds species were ‘hitchhiking’ across the state in the same containers as ornamental flowers and shrubs.”

The researchers found 54 nonnative weed and invasive species, with the five most common “hitchhiker” weeds being sticky chickweed (Cerastium glomeratum), hairy bittercress (Cardamine hirsuta), common groundsel (Senecio vulgaris), La Plata sandspurry (Spergularia plantensis) and birch (Betula sp.). Of the 54 plants that sprouted, only three were native to Alaska. “Based on the Alaska data, it is clear that container plants play a role in the spread of weeds that can be a risk to native plants and wildlife habitats,” Lee said.    

Researchers found that several variables affected the number of weeds found:

• Soil from balled or burlap-covered trees and shrubs held more weed seeds than vegetable, herb and perennial containers. 

• More weeds sprouted from soil-based mixes and mineral soil than from sterile, soilless potting mixes like sphagnum peat moss. 

• The grower or vendor made a difference. It was evident that several had superior weed control practices and sold plants with few if any weeds.

“Prevention is always the most powerful and cost-effective approach for combating weeds,” Lee said.  “We urge growers and retailers to make weed control a routine part of their operations and urge homeowners to be vigilant. If you spot weeds that sprout alongside your new container-grown plants, pull them before they produce seed and spread. It can make a real difference.”