Contain those weeds

Spilled growing medium can be a source for weed seed germination and growth.The most effective method of controlling weeds in container nurseries is to institute preventive measures. In general, the cost of preventing weed growth is substantially lower than the cost of managing weed infestations.
 
One theory is weed seeds are introduced into the nursery in potting mixes. This possibility seemed worth investigating based on an informal survey of nursery weed management practices in north Florida, and considering practices used to harvest pine bark, a widely used component of the mixes.
 
Fresh potting media was collected from three north Florida nurseries in February 2006 and stored indoors until June 2006. In early June, each substrate was put in 20 4-inch pots, and the 60 pots were placed in a greenhouse. Pots were irrigated as needed to keep the substrate moist.
 
Weed emergence was minimal — only three weeds were observed during the 16 weeks, including bittercress and chickweed. The study was repeated in late September 2006. Two weeds were observed after four weeks — woodsorrel and bittercress.
 
Interestingly, four of five weeds occurred in the substrate from one nursery. Japanese climbing fern was observed in several pots during the summer but the fern spores might have gotten into the greenhouse through open vents.
 
Bark and sand were not a significant source of weed seed introduction into the nursery.

Weeds of concern
Infestations of bittercress and common groundsel can be managed or controlled by applying herbicides that prevent their emergence, or at least emergence of most of them. Herbicides rarely provide 100 percent control of these species. Weeds scientists consider less than 90 percent control as excellent. The problem is 90 out of 100 means 10 weeds survive, 900 out of 1,000 means 100 weeds survive, etc. Weeds that do emerge will exhibit varying degrees of tolerance to the herbicide. Those that survive are still capable of flowering and setting seed and must be eradicated. Moreover, 90 percent control might not be acceptable for difficult-to-control species like bittercress and common groundsel.

Sanitation practices
Approaches to weed management need to be more ecologically oriented, in much the same manner that has been espoused for insect and disease management. 
 
Become familiar with your potential weed problems. This, in part, means following up on accurate identification through your extension agent, diagnostic lab or reference materials. Scheduling of preemergence herbicide applications must be based on when weed seed are likely to germinate under the weather and production conditions at your nursery. Note when weeds emerge in relation to the weather and the number of days or weeks after a plant is potted. Seed of some species don’t germinate for awhile because they contain inhibitors that must first be leached out via rain and irrigation.

Preemergence primer
Use a variety of preemergence herbicides with different modes/sites of action for weed control. Long-term use of herbicides with the same mode/site of action to control a particular weed could result in that weed becoming tolerant to those herbicides.
 
Preemergence herbicides usually need to be applied at specific intervals in order to maintain a high level of weed control. Weather extremes could alter these intervals. However, the number of times an active ingredient can be applied to a crop might be limited over a 12-month period.

Eclipta alba has escaped through the pot drain hole.
Scouting for weeds
Closely monitor newly potted plants. Under nursery conditions, weeds could emerge only a few days after potting. Newly potted plants are very susceptible to weed infestation due to ideal conditions like well-drained and moist potting media, exposure to the sun and good nutrient availability.
 
If weeds do emerge soon after potting, note the location. If weeds emerge at the base of the liner, weed seed were likely brought in with the liner media (or the liner or liner container deflected the herbicide away from that area). If weeds are in the new medium, weed seeds were probably blown or splashed in. 
 
Be especially vigilant of weeds with effective seed dispersal mechanisms. Bittercress and woodsorrel use “explosive dehiscence” to eject mature seed up to several feet, and potentially further if caught in the wind.

Keep it clean
Maintain weed-free zones, especially in and around potting areas, substrate storage areas, container beds, and propagation areas. Use mowing, living and nonliving mulches, nonselective herbicides and noncrop herbicides with long residual activity where appropriate.
 
When mowing vegetation surrounding a nursery bed or propagation area, direct mower discharge away from the bed or propagation area, use a mulching mower, or block the discharge chute so that clippings remain in the path of the mower. String trimmers can throw weed seed in any direction.
 
Check your shoes, socks and pant legs. Seed can easily become lodged in the mud and soil of shoes. Some seeds are especially adapted to cling/stick to passing animals, including you and your workers.
 
Pots to be reused must be thoroughly washed inside and out. Weed seeds are rarely visible and are often hidden in old potting substrate. Using warm, soapy water will help to remove seeds in soil and attached to the pots, but will not kill seed.
 
When purchasing liners or transplants from a new or unfamiliar supplier, consider an informal tour of the operation first to see how well the company manages weeds. In some cases, weed-seed-contaminated medium near the surface (about ¼ to ? inches) can be removed prior to planting liners, and new, clean medium can then cover up any seeds further below.
 
Remove spilled media from beds. Any medium that spills out of pots onto the container bed makes an excellent substrate for weed seed germination.

Eradicate weeds growing under pots, or in the pot drainage holes. 
 
Cover piles of potting media whenever possible. A water impermeable barrier as simple as black plastic will help keep weed seeds out of media and reduce loss of media and fertilizer with rain. And finally, include weeds as part of your insect, mite and disease scouting program. 

 
Jeffrey G. Norcini, a former University of Florida professor, is now owner of OecoHort LLC; www.oecohort.com. Alejandro Bolques is a horticulture and small farms extension agent, Florida A&M University; abol@ufl.edu. Robert Stamps is professor of environmental horticulture, University of Florida, Mid-Florida Research and Education Center; rstamps@ufl.edu.