You have heard the old adage that “an ounce of prevention is worth a pound of cure” and in high tunnel production that is sound advice. Pest management inside a high tunnel is particularly difficult, because there are few chemicals registered for use on crops grown in them. Also, the structure is vented to the outside, which allows the possible influx of pests on a regularly basis.
Weeds are a constant problem inside and outside of growing structures. Weeds compete with your crop for light, nutrients and water. They also increase the possibilities of insect and disease problems by harboring these pests in or around your tunnel. Finally, they can reduce air circulation within structures when they are vented.
Prior to Construction
Perennial weeds are best managed prior to construction of your high tunnel. If possible,
you should avoid placing your tunnel on a site with a high population of noxious
perennial weeds. Consider using a systemic herbicide a year or two prior to installing
the high tunnel to kill the vegetative propagules of such weeds.
An alternative strategy would be the repeated removal of top growth to deplete the stored sugars for regrowth thereby killing them. Tillage may sometimes aggravate the problem by chopping up the underground parts and spreading them, unless they are removed carefully. Crop rotations and cover/smother crops are additional strategies to consider, prior to installation, especially to manage annual or biennial weeds.
Once the tunnel is up, your options for control are limited. Cultivation or hand weeding can manage weeds within rows. Plastic or organic mulches are often used to suppress weeds within rows and long the edges of the tunnel. If you use an organic mulch be sure to use something that does not contain seeds; otherwise, you will create a new weed problem to fix. Consider using a landscape fabric where rows or edges need to be kept covered in the long term.
A vegetation-free strip should be maintained around the outside of your high tunnel. This will reduce the chance of weed seeds entering your production area and reduce sources for diseases and insects. Removing or mowing the vegetation are both good options, as well as applying a mulch such as landscape fabric, around the house to suppress any weeds.
Finally, if you do use herbicides on your farm, be careful to not allow drift to enter your high tunnel. Many herbicides can easily disperse and spray drift can damage your plants. If herbicides are being used elsewhere on your farm, it is best to close your vents to limit the potential of drift on your plants and/or be sure to be downwind of your high tunnel before applying a spray.
The best advice to preventing weed problems is to reduce them before erecting your high tunnel. Then, continue to keep any new weeds at bay to reduce this pest in your high tunnel production.