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Weed Control in Pumpkins and Winter Squash

Author: Marissa Schuh, Integrated Pest Management Extension Educator, University of Minnesota Extension. Reviewed by Annie Klodd, Extension Educator - Fruit and Vegetable Production


With their long season and spreading growth habit, pumpkins often present a weed control challenge.  Here are some important factors and considerations when working to manage weeds in pumpkins and winter squash. 


Waterhemp in a pumpkin patch. While a few sporadic weeds may seem insignificant, one average waterhemp plant produces about 250,000 seeds. Photo: Annie Klodd, University of Minnesota Extension.

First, know your weeds.

This is beyond knowing you have general issues with grasses or broadleaves.  Some cultural techniques and herbicide chemistries are more effective against some weeds than others.  Being familiar with the specific weed issues in the fields you are planting into will help you tailor your weed control program for success.  The row crops side of the University of Minnesota Extension has a good rundown of common Minnesota weeds (including some guides to seedling ID).


Cultural Weed Control

Rotation is important to keep soil-borne diseases like phytophthora and fusarium away, but can also be important as it allows for other crops that break weed cycles or have alternative herbicide chemistries available to be part of long-term weed management. For pumpkins and winter squash, a three year rotation is a good starting point.


Be careful when rotating with field crops, as many corn herbicides have a 18 or 24  month replant interval for specialty crops.  If you swap land with a neighbor, it is worth some research and a conversation to figure out how their weed control can work for both of you.


For agritourism farms, parking, attractions, and customer access to fields can make rotation tricky.  The increased popularity of sunflowers in recent years allows an opportunity to  diversify crop rotation, weed control, and income timing.


Mechanical Weed Control

Tillage, especially before planting, is important for starting weed free.  Both organic and conventional growers should consider preparing the field 2-4 weeks in advance, letting a round of weeds flush, then killing these weeds with very shallow tillage, flaming, or a burndown herbicide.  This is called the stale seedbed method.


Another important time for mechanical weed control is before weeds set seed.  Many weeds produce thousands of seeds that can survive in the soil for long periods of time (some species over a decade), depending on the species and soil conditions.  For example, the average common lambsquarters plant sets over 70,000 seeds, eastern black nightshade produces 10,000, and marestail over 200,000.  Hand-removing weed escapes before they set seed can prevent years of future weed problems.

     

Pre-Emergence Herbicide Options
For conventional growers, pre-emergence herbicides are a cornerstone of good weed control.  These chemistries create a chemical barrier in the soil that recently germinated seeds cannot break through.  This means young pumpkin plants start the season with minimal competition from weeds, allowing them to become well-established and get in a few weeks of solid growth.  Once the pre-emergent herbicide is no longer active, the larger plants will be better able to out-compete mid- and late-season weeds.  Labor for hand-weeding is expensive and hard to come by, and post-emergence chemistries have limitations and risks to the main crop, making pre-emergence herbicides worth considering.


Comparison of a row of squash treated with no pre-emergent herbicides vs a row treated with a pre-emergent herbicide. Photo: Bernie Zandstra, Michigan State University.


For all herbicides, remember the label is the law.  It is worth giving the following herbicide labels an extra close review, as there is a lot of nuance as to when they can be used and what varieties they can be used on.


Command 3ME (clamazone)

  • Labelled for: Winter Squash 

  • Product per Acre: 10.7 - 32 fl. Oz. (dependent on soil texture)

  • Weeds Controlled: Grasses and some broadleaves: common lambsquarters, common ragweed, velvetleaf (but not pigweed)

  • Application Timing: Apply after seeding transplanting.  If using plastic, apply just prior to transplanting to row middles.

  • Notes: Do not use Jack-O-Lantern Pumpkins, Turban squash, Golden Delicious, and other C. maxima species with pink or burnt orange coloration at harvest. Do not use under plastic.


Curbit (ethalfluralin)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 3-4 pts/acre (rate dependent on soil texture)

  • Weeds Controlled: Grasses, some broadleaves

  • Application Timing: Apply to soil surface within 2 days after seeding or before transplanting as banded spray between rows.

  • Notes: Needs 0.5 inch of rain within five days for activation. Heavy rains may cause crop injury. Do not apply under plastic mulch. Cool temperatures can cause crop injury.


Dual Mangum (s-metolachlor) *

  • Labelled for: Pumpkins, Winter squash

  • Product per Acre: 1-1.3 pints (depends on soil texture)

  • Weeds Controlled: Grasses, pigweed, nutsedge, nightshade 

  • Application Timing: Apply after seeding and before emergence.

  • Notes: See indemnified label note below.


Reflex 2L (fomesafen) *

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 0.5-1pt. (max rate depends on location in state)

  • Weeds Controlled: Broadleaves; common ragweed, lambsquarters

  • Application Timing: Apply after seeding but before emergence. For transplanting, apply 7 days pre transplant

  • Notes: Restrictions vary by location in state. May cause crop injury, especially if soils are cool. Butternut squash is very sensitive. See indemnified label note below.


Sandea (halosulfuron)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 0.5-0.75 ounce

  • Weeds Controlled: Nutsedge, Broadleaves

  • Application Timing: After crop seeding but prior to soil cracking, or 7 days before transplanting.

  • Notes: Heavy rain (0.75 inches or more) and cool weather can cause crop injury. Or, can be used post emergence as directed spray between rows, see label.


Strategy (ethalfluralin and clamazone)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 2-6 pints (for squash, use lower rates in range)

  • Weeds Controlled: Broadleaves and grasses

  • Application Timing: Apply within 2 days after seeding. If transplanting, as banded spray between rows.

  • Notes: Needs 0.5 inches of water within 5 days for activation, and if not possible, shallowly cultivate. May cause injury in cool weather.


*These are Minnesota 24(c) labels that have been approved through 2025. If using, you will need to create an account on Syngenta’s website to accept and view the indemnified label.  Label should be in your possession at the time of application. Please watch this video from the MN Fruit and Vegetable Growers Association to learn about these products and the indemnified labels."


Post-Emergent Herbicide Options 
Remember, these will be the most effective when applied to weeds that are less than 6 inches tall (though the smaller the better).  Many of these require specialized spray equipment or technique, as they will damage pumpkins if they touch them.  Some of these can be used as a burndown pre-plant, and if using a product in that way, keep an eye on application limits. 

Aim (cafentrazone)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 0.5 - 2 fl. oz.

  • Weeds Controlled: Broadleaves, especially lambsquarters, morning glory, nightshade, velvetleaf, pigweed (if smaller than 3 inches)

  • Application Timing: Apply minimum 1 day before transplanting, or 7 days prior to direct seeding. Apply between rows with a hooded sprayer

  • Notes: Add 1 qt. COC (1% v/v) or 0.5 pt. NIS per 25 gal. of spray solution (0.25% v/v). Weeds must be actively growing and less than 4 inches tall. Do not exceed 6.1 fl. oz. per acre per season. Maximum 6.1 fl. oz. per acre per year.


Glyphosate Products

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 0.75-3.75 lbs. acid equivalent (ae) per acre (low rate for annuals, high for perennials)

  • Weeds Controlled: Annual and perennial broadleaves and grasses

  • Application Timing: At least 3 days before planting/transplanting. Post-plant, apply between crop rows with hooded or shielded sprayer.

  • Notes: Remove residue from plastic mulch before planting.


Poast (sethoxydim)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 1-1.5 pt.

  • Weeds Controlled: Grasses, especially annual grasses

  • Application Timing: When grasses are actively growing.

  • Notes: Add 1 qt. COC per 25 gal. of spray solution (1% v/v). Spray on actively growing grass. Do not exceed 3 pts. per acre per growing season.


Sandea (halosulfuron)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 0.5 oz.

  • Weeds Controlled: Broadleaves

  • Application Timing: When crop has 3-5 leaves, or 14 days after transplanting. Apply before female flowers appear. Direct spray between rows if possible. Do not apply more than 2 applications of 1oz/acre or 2 oz/acre per 12 month period.


Select Max (clethodim)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 9-16 fl. Oz. (higher rate for perennial grasses)

  • Weeds Controlled: Grasses

  • Application Timing: When grasses are actively growing.

  • Notes: Select Max at 9-16 fl. oz. per acre with 1 qt. COC (1% v/v) or 0.5 pt. NIS per 25 gals. of spray solution (0.25% v/v). Do not exceed 64 fl. oz. per acre per season. Wait at least 14 days between applications.


Scythe (pelargonic acid)

  • Labelled for: Pumpkins, Winter Squash

  • Product per Acre: 2-10 gal.

  • Weeds Controlled: Small emerged annuals

  • Application Timing: Before planting, or after seeding before crop emergence, or as directed spray between rows.

  • Notes: Good coverage needed for control, as only has contact activity. Use 75-100 gals. total spray solution per acre.  Check with organic certified before use.


For more details on all products mentioned, see the labels. For full list of recommended products available, see the Midwest Vegetable Production Guide.

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