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Weekly Vegetable Update 6/22/2023

We'll spare you a note about air quality, heat, and drought -- you all know it's rough out there. Read on for notes about crop growth progress and common issues, as well as information about pests to look out for, holding lettuce, lambsquarters, and solarization.

Crop updates

Brassicas: We’ve heard some concern from growers that their brassica crops are not producing heads. It’s a bit early to be concerned - the growing season started late, and while it feels late in the season due to the heat, it’s still early enough that we’re not too concerned that heads are not forming. We’re also reaching the point in the season where cabbage caterpillars are reaching thresholds for treatment. Treatment thresholds for Minnesota are…

For broccoli:

Plants in transplant through first curd stage

  • Imported cabbageworm, cabbage looper - 40% of plants with larvae.

  • Diamondback moth - 20% of plants with larvae 

Plant in first curd to final harvest

  • Imported cabbageworm, cabbage looper - 10% of plants with larvae.

  • Diamondback moth - 10% of plants with larvae.

For cabbage:

Transplant to cupping stage

  • 50% infested with 5 or more diamondback moth larvae.

  • 30% plants infested with 1 or more medium to large imported cabbageworm larvae.

Plant in cupping to harvest stage

  • 10% of plants infested with 1 or more diamondback moth larvae.

  • 10% plants infested with 1 or more medium-large imported cabbage worm larvae and cabbage looper eggs or larvae.

Cucurbits: Usually this time of year we’re writing extensively about cucumber beetles. We’ve actually seen very little cucumber beetle damage this year, but it does tend to be spotty, and so we may just be visiting the wrong farms.  If they're bad on your farm, read about mid-season management tactics here.

Based on degree days, we are likely going to see squash vine borer emerge and start to lay eggs.  If you’re out working on the farm and you think you see some sort of orange wasp flying around your vine crops, take a minute to take a look. Adult squash vine borers are day-flying wasp mimics, see if you see the insect land to confirm its identity – when the moths land, they look pretty different  than wasps.

Squash vine borer are about a half inch long with orange bodies and clear wings that appear black at rest. Photo: Marissa Schuh, UMN Extension.

If you’re seeing adults and have had losses from this pest in the past,  you can look for eggs and larvae (and the sawdust their burrowing causes)  in the crowns of plants. For more info about sprays for squash vine borer, see the Midwest Vegetable Production Guide.

Lettuce: We’re reaching the part of the season where lettuce tends to get stressed. Many growers harvest their final early season lettuce right about now, and then take a break until conditions cool down a bit. Some growers use shade cloth for lettuce, but research from Dr. Cindy Tong in 2018 and 2019 showed that at least for heat tolerant varieties of lettuce, shade cloth actually reduced yields (measured in fresh weight) and reduced the sugar content of the leaves without impacting bitterness. In her trials, the varieties Sparx and Salvius grew well in unshaded conditions in the heat of the summer. Read more about her experiment here. She also has a site about building shade hoops including a cost analysis. 

If you are concerned about your current lettuce crop bolting and it’s harvestable, lettuce can be kept for about 2 weeks, ideally at 32 degrees fahrenheit and 95% humidity. 

Tomatoes: We’ve seen a few less than thriving tomato plants lately. At one farm, we noticed that every ten plants or so was really struggling, with dead leaves throughout the plant, and some random phosphorous deficient plants throughout the field. The field was definitely not deficient in phosphorus, and the distribution of symptoms was too random to be a significant nutrient or disease issue. We determined that the symptoms were likely a result of inconsistent irrigation from clogged emitters, and stressed tomato plants that were transplanted late and didn’t get their roots established well enough in time for the heat stress that they endured in their first few weeks in the field. 

The season has been hard on plants -- lots of abiotic stresses are occurring that can look like disease. Photo: Natalie Hoidal, UMN Extension.

We’re also seeing quite a bit of leaf curl. This is a response to heat, and varies quite a bit from variety to variety. Keeping tomatoes well irrigated is the best tool we have to prevent heat stress at this point in the field. In tunnels, maintaining really good ventilation is key. If you haven’t done so, installing a gable vent at the top of your high tunnel is pretty quick / easy, and it makes a big difference for letting hot air out of tunnels. 

Leaf curl is a heat response that some varieties do more of. Photo: Natalie Hoidal, UMN Extension

Garlic and onions: With the hot, dry weather ahead, now is a good time to start regularly scouting for onion thrips, which can significantly reduce yields of onions and garlic, and are hard to see without looking for them.  Managing these pests with insecticides is super tricky, as it appears the order products are applied in during the season has a huge impact on how effective they are.  Conventional growers curious about product selection should dig into work done in Michigan on the topic.  On the organic side, information is a little more limited.  Trials in Wisconsin have found that no organic products significantly reduced thrips populations in onions in years with typical and low population years. 

Asparagus: As harvest winds down, don’t forget to keep an eye out for diseases in the patch.  In newly planted fields, keep an eye out for asparagus rust and move impact spears or fronds. Purple spot is more common in fields where last year’s ferns aren’t removed. For more information, see “Asparagus pest management.”

Weird looking plants? Check for two spotted spider mites and thrips

On top of all the plant stress symptoms we are seeing from heat, large transplants, and draught, this type of weather also promotes a couple of pests that love hot weather – thrips and spider mites.  Their damage often confusing to see, keep an eye out for the types of damage pictured below. If you see damage, take a closer look, like really close, both these insects are “you might need a hand lens to see me” small.  Spider mites are circular and leave behind dense webs, thrips are oblong and yellow.

Two-spotted spider mites cause “stippling”, or a series of white spots.  This can be densely packed or more sporadic. Whitney Cranshaw, Colorado State University,

Thrips can cause silver patches on leaves, Whitney Cranshaw, Colorado State University,

If things cool or we get rain, these populations can drop pretty quickly. For pesticide recommendations, see the Midwest Vegetable Production Guide.


As extension educators visit farms, we find ourselves saying “It’s a great lambsquarters year…but then again, so is every year.” Why is this weed so common? Each plant makes thousands of long-lived seeds (like, scientists have found viable lambsquarter seeds in medieval ruins long-lived).  Things to think about when trying to deal with this weed…

  • They are heavy feeders of NPK.  Organic systems will always struggle with having enough N, don’t discount how much N weeds can use.

  • These seeds are long-lived and numerous, but don’t have lots of energy for seedlings to use. Seedlings are easy to kill with light tillage, or even just when walking over them.

  • Seeds also don’t provide a lot of energy for pushing up through the soil or a mulch. Lambsquarters seedlings rarely are able to germinate if buried below 1.2 inches of soil or 2 inches of loose straw.

  • Prevent seeds from forming by mowing aisles or field edges. Seeds from this weed are good at moving around with equipment, shoes, water, and even livestock.  It takes 2-3 weeks for lambsquarters to go from flowering to seed production.

  • Lambsquarters in onion and cabbage fields have been found to produce 30,000 to 370,000 seeds, so removing big plants by hand will help with long-term management. 

Where the landscape fabric stops, the lambsquarters start. Photo: Marissa Schuh, UMN Extension.

Weed tree management 

Another weed occurrence we have noticed is weedy trees in fields. The last few years of drought (and before that, incredibly wet years) have put a lot of stress on trees.  Some tree species respond to stress by putting out more seeds before death, and this increase in seeds can mean more weedy tree species.  Don’t neglect pulling these when they’re small.

Solarization vs. occultation for bed flipping and cover crop termination

Around this time of year we see growers using occultation or solarization (aka the use of opaque or clear tarps to kill the plants underneath) to terminate early season cover crops and to flip beds. While solarization is touted as faster and more efficient on many online publications, these publications often come from drier places. While we haven’t done comprehensive research on this in Minnesota, many farmers have found that opaque plastic is more effective than clear plastic. One hypothesis is that we have more soil moisture (even in dry years) than some of the other places where this research has been done, and so greenhouse effect of the clear plastic is less effective at killing plants than simply depriving them of light, which is how opaque tarps kill vegetation.

Clear plastic isn't quite getting the job done here. Photo: Natalie Hoidal, UMN Extension.


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