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Weekly vegetable update of 2025 - May 28, 2025

Cool temperatures have made for comfortable planting weather, but we’ve seen quite a few cold stressed plants this week. The week ahead looks ideal for planting: warmer, drier weather means it’s time to plant all of our warm-season vegetables outdoors. This week you’ll want to keep an eye on: cutworms, high tunnel humidity, and preventing plant damage when using landscape fabric and plastic mulch.

Cold stressed plants

Heat-loving plants like cucumbers, sweet potatoes, bail, and peppers can experience significant cold stress when they’re moved outside, even if temperatures don’t technically reach freezing. While it can be tempting to start planting in early to mid May when the weather is warm and sunny, we so often have a cold snap towards the end of May that it’s usually worth waiting until early June to plant our heat-loving veggies. The basil and sweet potatoes in the photos below show classic signs of cold damage. They will likely survive, but they are starting the season at a disadvantage.

Basil seedlings with dessicated leaves

Sweet potato seedlings with dessicated leaves

Photos: Cold stressed basil and sweet potatoes. Photos: Natalie Hoidal

Smoke in the forecast

Farmers in Northeast Minnesota will have to navigate smoky air today, and it's looking like wildfires in Canada could lead to air quality alerts all over Minnesota throughout June.  Best practices for farming in poor air quality include: 
  • Wear an N95 respirator when you're working outdoors, and train your team to wear respirators properly. You can also wear a respirator with even more filtration: N99, N100, R95, R99, R100, P95, P99, and P100. Surgical masks and bandanas do not protect you from the small particles created by wildfires and ozone. Discard yomprur respirator after 8 hours and start each day with a new one. 
  • Protect your eyes: use eye drops to keep your eyes lubricated, and if you wear contacts, consider wearing glasses on days with poor air quality. 
  • Set up a clean air area. When the air quality index reaches 101, reduce your time outside and take frequent breaks. Designate a break room where you can keep windows and doors closed and put a filter in the break room. You can purchase a HEPA air filter, or make your own using furnace filters and a box fan
  • Read more about heat and air quality safety for farmers. 
 
Air quality forecast map from the Minnesota Pollution Control Agency showing unhealthy air for sensitive groups in Northeast Minnesota. Check the air quality forecast at https://www.pca.state.mn.us/air-water-land-climate/air-quality-forecast

Cutworms

We’ve heard so many reports of cutworm damage this spring, especially around the metro and south-central MN. Cutworm larvae from moths that laid eggs in April are now large enough to do significant damage to your young plants. We’re also seeing some armyworm infestations in southern MN. Some strategies for managing cutworms include:

  • Good weed control in fields that you are getting ready to plant
  • Tilling a field before planting can help expose and kill larvae
  • Leave a 3-4 foot area of bare soil around valuable plantings to make it less attractive for cutworms to crawl across.
  • On a small scale (such as in a high tunnel), using foil or cardboard collars around transplants can create a physical barrier that stops cutworm larvae from feeding on plants
  • Pesticide applications are most effective between midnight and dawn since cutworms are active at night. If caterpillars are already large enough to cut plants, pesticides will be less effective.
  • For recommended insecticides (both OMRI approved and not) in our region, refer to the Midwest Vegetable Production Guide. Search for “caterpillars” when using the online guide to find products for cutworms.


Landscape fabric & heat tunnels

We’ve seen some very loosely applied plastic mulch and landscape fabric this week. These are valuable tools for weed management, but if they are not spread tight over the soil and staked down really well, they can damage transplants. Temperatures under black plastic can get extremely hot. When the plastic blows in the wind, the hot air underneath gets blown up through the holes where your plants are, and can cause stem girdling. As temperatures begin to warm up, now is the time to make sure your plastic is pulled tight and sufficiently secured. Young transplants with tender stems are especially susceptible to damage.


A tomato stem that is thin and worn down near the base

Photo: Girdled tomato stem from landscape fabric. Natalie Hoidal

High tunnel humidity

I’ve been in a couple of high tunnels this week that were so humid, condensation was dripping down from the roof onto the plants below. This is an important time to take stock of high tunnel humidity, as these conditions will make disease management very tricky as your high value summer crops begin to grow. If you’re seeing significant condensation in your tunnel, it’s worth taking some time to figure out how to add a vent or two along the top of your tunnel (these are called “Gable vents”) if you don’t have them already, and to consider adding a powered fan. While roll-up or roll-down sides allow cool air into the tunnel, you need a place near the top of the tunnel for hot air to escape. This could be a passive vent like a gable vent, or an active fan system. Read more about high tunnel ventilation.


Diagram showing the outline of a high tunnel with blue arrows indicating the movement of cold air into the tunnel, and red arrows indicating the movement of hot air to a vent at the top of the tunnel.

Images: High tunnel air circulation diagram by Natalie Hoidal & Lindsey Miller, Gable Vent at Food Farm (Photo Natalie Hoidal), and an electric powered exhaust fan (Photo Chris Callahan, UVM Extension).

Crop updates

  • Sweet corn, beans, melons, and squash: It will finally be warm enough in most of the state to direct seed these warm soil loving plants this week. Remember that plants in the cucurbit family have extremely sensitive roots, and so if you’re transplanting these crops, avoid root disturbance as much as possible. While we haven’t seen cucumber beetles yet, they tend to emerge right when growers start planting, so take precautions with row cover, and get ready to start scouting. 
  • Cole crops: We’ve seen some very healthy plantings of broccoli, collards, cabbage, kale, etc. As temperatures warm up, expect to start seeing flea beetles. Take some time this week to review your flea beetle management strategies.
  • Tomatoes and peppers: If you’re transplanting this week, your plants will be transitioning from pretty cool, overcast greenhouse conditions to hot sun. Any time we transition quickly from a cool period where plants are indoors, to a hot sunny period during transplanting, it’s worth giving your plants a little bit of extra hardening-off. You can use physical agitation (e.g. brushing your clean hand over plants, moving a broom handle through your transplant trays, a strong fan to create some wind) to help strengthen the stems ahead of transplanting.
  • Spring high tunnel crops: We’re just starting to see some beets and carrots maturing in high tunnels as planting of these crops continues in the field. Some growers are harvesting their second successions of spring lettuce crops. We see so many high tunnel sitting empty this time of year, awaiting tomatoes or cucumbers, but we know that the most profitable high tunnel growers take advantage of growing greens, Brassicas, and even root crops in the shoulder seasons. 

 

Author: Natalie Hoidal


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