Keeping Weeds out of Your Operation

by Cheryl Wilen

Weeds can easily infest a nursery or greenhouse if good sanitation procedures are not followed.  In general, most problematic weeds are annuals and are introduced as seeds although some perennial weeds such as yellow nutsedge can be introduced as tubers if mineral soil or sand is used as a potting mix component. 

Common nursery weeds are those that have seeds that are easily picked up by the wind such as common groundsel and northern willowherb and those that have a special mechanism that promotes the spread of the seeds. These include woodsorrel and bittercress species. 

It is important to prevent these easily spread weeds from entering the growing area and to quickly remove them well before flowering.  For wind-carried seeds, prevention starts by cleaning up the areas in and around the beds.  Gravel beds should be floated between crops and nursery cloth should be inspected for holes and spilled soil.  Removing the soil is a particularly important sanitation task as weed seeds can easily become established there and root into the nursery cloth, creating more holes when removed (fig. 1).  Herbicides can be used to prevent weeds but care must be taken to choose ones that have high Koc values (herbicides that are strongly adsorbed to soil particles) or low solubility to reduce runoff or leaching. 

Within nursery containers, herbicides or mulches can also suppress weeds seed establishment. We have found that mulches are most effective in 5-gallon and larger containers, and a 1-inch layer of coarse mulch (with a particle size about 1/4 to 1/2 inch) is generally needed to get good weed control. When herbicides are used, the first application should be made as soon as possible after setting out the plants. The longer the plants are in the open unprotected from weeds blowing in, the harder it is to control weeds through the whole plant growing cycle.

For weeds that spread shorter distances by “popping” from pods such as woodsorrel and bittercress, the same precautions should be taken. However, these types of weeds often have rough seed coats that stick to plastic materials, including pots and irrigation tubing, and other surfaces (fig. 2). Where these weeds are found, part of the sanitation routine should be to use new pots or thoroughly clean used ones and wipe down benches and tubing.  Of course, all areas should be monitored and weeds removed before they flower as the first step of prevention. Start clean and stay clean.

Fig. 2. Oxalis seeds can stick on plastic such as this watering can (left) and on other surfaces such as this greenhouse pole (right), serving as a source of inoculum for nursery plants. Photo: C. Wilen.

In greenhouses, weeds around the doors (fig. 3) should be controlled, not only because seeds can get into the greenhouse and infest the pots, but also because weeds can harbor pest insects or plant pathogens that can easily move into the greenhouse.  For similar reasons, weeds under benches (fig. 4) or growing along the inside walls should be cleaned up.  Often the reason weeds proliferate in these areas is due to overwatering, resulting in overly wet and often highly fertilized places that promote weed growth.  Modifying irrigation method and amount and improving drainage will greatly reduce weed growth.

Pulled weeds should be removed from the growing area and placed in covered trash containers.  Some weeds will root from stems left on the ground and flowers may continue to mature on some plants (see video at http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=17079) which may produce viable seeds.

Other areas where sanitation is key include piles of potting mix or their individual components.  Piles being stored for custom mixing tend to be great sites for weed seeds to land on, grow on, or even just remain dormant. This problem can be reduced by controlling nearby weeds and by using a tarp or plastic to protect the piles. Locating the storage and mixing area away from areas where weeds are growing will also help.  Try to use bulk potting mix or components quickly; the longer a pile sits the more chance there is that it will collect weed seeds.

Finally, observe liners for a few days after receiving or transplanting. If possible, keep them in a holding area to make sure that new weeds are not introduced from these liners.  While most liners come clean, there are occasions where a change in environment will promote the growth of some weeds that were not obvious in the liner production stage.

Adjusting irrigation to reduce runoff, using mulches and appropriate timing of herbicide applications will greatly reduce weed pressure. Concurrent use of sanitation methods, such as being diligent in weed removal in and around beds and greenhouses, under benches and near potting media storage, will contribute to a more sustainable weed management system. 

Cheryl Wilen is UC Cooperative Extension Area Integrated Pest Management Advisor, Los Angeles, Orange and San Diego Counties, and UC Statewide IPM Program; and Endemic and Invasive Pests and Diseases Initiative Leader.