Weed Seeds: Are They a Problem?
by Steve Peters

An essential requirement for producing high-quality vegetable, flower, or herb seed is to minimize (and in some cases completely eliminate) noxious weed seeds that may be present in the seed lot. A "noxious weed" is defined as any persistent, nonnative, invasive species that causes economic or environmental harm, or is harmful to human health. When noxious weeds are introduced to an ecosystem, biodiversity is often reduced, natural succession is disrupted, hydrologic and soil characteristics are altered, and valuable indigenous plant (and animal) species may be threatened or eliminated. This invasiveness is characterized by rapid vegetative growth, high reproductive rates, and prolific seed production.

The USDA's Animal and Plant Health Inspection Service & Plant Protection and Quarantine Program (APHIS-PPQ), in conjunction with the plant-health agencies in each state, has established a list of these noxious weeds and is charged with regulating the shipment and distribution of agricultural seed to prevent the spread of these unwanted species. The Federal Seed Act (FSA) requires any agricultural seed shipped across state lines or to a foreign country, in one pound quantities or greater, to bear a label that states the kinds of noxious weed seeds present and the rate of occurrence of each. These weed species are considered "restricted," meaning that although they are invasive, they can be tolerated at a certain low level (set by each state). Other more persistently invasive plant species, however, are completely "prohibited," meaning that not even one seed of these plants will be allowed in a seed lot, and that it is illegal to sell seed that contains these kinds of weed seeds. For more information, go to www.ams.usda.gov/lsg/seed.htm. Click on "FSA and Regulations" and then click on the 2005 State Noxious-Weed Requirements—Complete Publication (States and Appendix) to see what weeds are restricted and prohibited in your state.

Keeping noxious weed seeds from contaminating agricultural seed is a particular challenge for organic seed farmers because they don't use chemical herbicides. When the desired seed crop is ready to harvest, it is common for nearby weeds to be "going to seed" at the same time. Mixing these weed seeds with the seed crop becomes a distinct possibility. For example, the restricted weed crabgrass (Digitaria sanguinalis) is often growing alongside fields of lettuce. Not only can the seeds of these two species mature simultaneously, but the size and color of the crabgrass seed is nearly identical to certain lettuce seed, which makes it nearly impossible to separate them during cleaning. A few years ago this was recognized as a problem for the Seeds of Change lettuce seed growers. Now our growers routinely mow the crabgrass on the borders of their lettuce fields, and pull out by hand any individual crabgrass plants within the lettuce rows before they go to seed. This problem with crabgrass has been largely eliminated; if it does occur, we do not sell contaminated lots in which the crabgrass seed levels are too high (above 27 seeds per pound of agricultural seed). Although we have screen cleaners, gravity tables (separating seed by weight), and optical separators (removing seed by color differences) that can remove unwanted seed, it is clearly more effective to prevent weed-seed contamination in the field. One of the most difficult weeds to control, field bindweed (Convovulus arvensis), is on the prohibited list in all fifty states. We advise seed growers to plant bindweed-infested fields with perennial crops or plant successive crops of winter rye and buckwheat for two years or more to smother the bindweed. Fortunately, if we detect bindweed seed in a particular seed lot, it can very often be removed during seed cleaning.

Over the past few years there has been an increasingly heated debate regarding the extent of the threat that these invasive species pose to the environment. In 1999 the Federal Government set up the National Invasive Species Council (NISC) (www.invasivespecies.gov) for the purpose of developing a screening system to determine what plant species are approved for distribution and sale. In reaction to these restrictions there is a growing group of activist citizens who are bitterly opposed to this kind of regulation, which, they argue, would be hugely expensive to monitor and test and would greatly restrict the movement of valuable plant material throughout the world.

This is an extremely contentious and complex issue. Most nonnative species are, in fact, not invasive. Furthermore, some native or indigenous species exhibit invasive tendencies. Nevertheless, the damage done to wilderness areas, in particular, by relatively few nonnative species, is of considerable concern. It must be recognized, however, that the invasive nature of these plant species is not the cause of the damage but merely a symptom of human carelessness and abuse of the land. Simply eradicating invasive plants (if that is even possible) may have even more negative consequences. For example, in the American Southwest the salt cedar (Tamarix spp.) has completely altered the riparian systems, and has outcompeted the native cottonwoods and willows. This is primarily from human-induced low water tables, increased salinity, and the reduction of natural flooding. Eliminating the salt cedar, however, could have devastating effects on the native willow flycatchers, which preferentially nest in the salt cedar.

The USDA has compiled an extensive annotated reading list for understanding and managing invasive plants in wilderness areas. This list is available at www.fs.fed.us/rm. Click on "search online publications" and enter the keywords "invasive plants," the publication is entitled "Linking Wilderness Research & Management – volume 4." There are no quick fixes or easy solutions to the issue of invasive species. The most effective strategies will require, at the very least, a regional approach over many years and will involve many sectors of our community. We can start by developing a greater understanding of our own ecosystems and of the role that each plant plays within it.

Steve Peters
Commercial Seed Manager