Weeds can be a serious problem by diverting moisture and nutrients from the plants you are growing intentionally. There are numerous options available to gardeners for effective weed control, and chemical herbicides should only be considered as a last resort due to their potential to harm other beneficial plants and introduce toxic chemicals into the ecosystem. Still, sometimes a weed problem is serious enough that it must be controlled with a herbicide. If you find yourself in this situation when growing pumpkins at home, you can design an effective herbicide treatment in a few basic steps.
Herbicides, pesticides, insecticides and other such chemicals are regulated on a state-by-state basis, so some or more of the chemicals recommended here may not be available in your area. Common "pre-emergent" herbicides -- those that are applied preventatively before weeds are observed -- approved for pumpkins include clomazone, ethalfluralin, bensulide, halosulfuron and others. Post-emergent herbicides -- those applied after weeds are seen -- include glyphosate, paraquat, sethoxydim, clethodim and others. Again, herbicide availability is highly region specific, so contact a local university extension service with questions about which herbicides are approved for use on pumpkins in your community.
Arguably a more important consideration than what herbicide you use is how you apply it. Always meticulously follow the application instructions printed on the herbicide's packaging. In general, pre-emergent herbicides are always applied when pumpkin plants are first planted in the ground or before the next growing season. Herbicide treatments should also be combined with natural and biological weed control, such as hand removal of weeds.
Probably the biggest obstacle to successful herbicidal weed control facing gardeners is the tendency of weeds to develop a resistance to the herbicides used. Not only does this not remove your weed problem; it often compels gardeners to use more and more herbicides, thus increasing the health and environmental effects of their use. To manage weed resistance to herbicides, rotate herbicide applications with non-chemical control methods over successive growing seasons, rotate applications from growing site to growing site and avoid using the same herbicide on the same site two seasons in a row.
Other Weed Control Measures
Natural and biological control methods also show a high degree of effectiveness in weed control around pumpkins. Remove weeds by hand in 10-day intervals before planting pumpkins -- this will prevent a weed from becoming established at the growing site. If possible, pull weeds out of the soil when they are in the cotyledon stage and dig them out from as shallow a position in the soil as is possible to prevent the distribution of weed seeds across the soil.
- Purdue University Cooperative Extension Service: Managing Weeds in Pumpkins and Other Cucurbits; Steve Weller
- University of Illinois Extension; Weed Control in Illinois Pumpkins; Elizabeth Wahle
- University of Tennessee Biosystems Engineering & Soil Science; Evaluation of Herbicides for Broadleaf Weed Control in Pumpkins; R. Allen Straw & Darren Robinson