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Weekly Vegetable Update 5/20/2021

Author: Natalie Hoidal, UMN Extension educator, local foods and vegetable crops  

If you're seeing interesting things in your fields, need help identifying problems, or just want to share photos, we'd love to hear from you! Growers can reach out directly to me any time at, and you can submit questions and diagnostic help here.

Vegetable weather report

After weeks of extremely dry weather, growers across the state are finally getting some much needed rain. While most of the state is showing multiple days of rain in the forecast, the total amount of rainfall will be fairly limited. Over the next 7 days Southern Minnesota is expected to receive an inch or less of precipitation. Growers in far Northern Minnesota can expect more like 2 inches.
7 day precipitation forecast from
Temperatures in far Northern Minnesota are still likely to dip into the mid 30's later this week, but for most of the state nighttime lows are projected to stay in the 40's, and even 50's in the Southern part of the state.
Soil temperatures are warming very quickly. In many parts of the state, temperatures are reaching the mid-60's, which is ideal for planting and transplanting most warm season crops. The Minnesota Department of Agriculture publishes daily updated soil temperatures for major field crop growing regions of the state. Data is compiled from MDA managed stations in the Southern half of the state, and NDAWN managed stations in the northern half. As such, the data is presented slightly differently. The following tables provide a snapshot of soil temperatures across the state. For more detailed info, see

Soil temperatures from MDA stations, data collected to 6’’ depth

Station location

Nearest major town

Coldest soil temperature in the last 7 days (approx.)

Temperature on May 18, 2020

Blue Earth

Albert Lea (Iowa border)

50.5 º F

55.5 º F



54 º F

67 º F


St. Peter / Belle Plaine

52 º F

68 º F


Wahpeton / Elbow Lake

52 º F

63º F

Soil temperatures from ND-managed stations in MN are collected to a depth of 4"

Station location

Nearest major town

Average turf soil temp on May 19  (4’’)


Fargo / Moorhead

63º F


Thief River Falls

55 º F


Roseau (Canada border)

61 º F

Crop updates

  • Cole crops: Most farmers have planted / transplanted their first couple of Brassica successions at this point. We had a glitch in our degree day models this week, so we'll have an update next week. But for now, we can assume that cabbage maggots will be flying if not now, very soon, in southern Minnesota, and seedcorn maggots are active. Put out your row cover if that's a strategy you use, or plan to manage these insects in other ways (for insecticide recommendations see the Midwest Vegetable Production Guide).
  • Sweet corn: Soil temperatures are reaching 60-65 degrees throughout much of the state, which translates to ideal sweet corn planting / transplanting temperatures.
  • Lettuce: Many farmers are making their last cuts of greenhouse lettuce this week to make way for tomatoes, cucumbers, etc. Meanwhile, the very first successions of outdoor lettuce are maturing this week.
  • Carrots and beets: Continue to provide irrigation through germination, especially if your soils are prone to crusting. Hot weather after rainfall can lead to crusting in clay soils, which can prevent germination and sever recently germinated seedlings. Carrots and beets, being very small-seeded crops, are quite susceptible.
  • Onions: Onions seem to be doing well. Onion seedlings can be effected by seedcorn maggot, so keep an eye out for damage.
  • Garlic: Garlic is beginning to enter the bulbing stage, at which point it becomes very susceptible to dry soil conditions. Make sure that your soil is not dropping below 50% water holding capacity (more about irrigation monitoring below). At this point, avoid applying more nitrogen.
  • Asparagus: Asparagus harvest is well under way.  
  • Potatoes: are starting to emerge. I've now received two reports of potato beetle emergence.
    Potato beetles have now been spotted on two farms in Minnesota. Photo: Ward Upham, Kansas State University,
  • Peas are establishing well with no reported problems so far. 
  • Cucurbits: Cucumbers and melons are being transplanted, and growers are starting to direct seed squash and pumpkins. We've seen some cold damage in pumpkins, which can look like yellowing of the leaf margins (this could also be a nutrient issue, but symptoms are showing up very suddenly after transplanting). Keep in mind that cucurbits are extremely sensitive to root disturbance, so handle your transplants carefully. If you have two seedlings in a cell, it is usually not worth trying to break them up.
  • Tomatoes: Tomatoes are again all over the place, ranging from the flowering stage in tunnels, to just being transplanted in both tunnels and the field. We've seen a few reports of diseased tomatoes already, so make sure you're scouting as you transplant, as well as regularly after that. We've also seen some potential cold damage symptoms in tomatoes that can look a bit like a nutrient deficiency (yellowing along the leaf margins).

Problems in the field and things to anticipate this week

Irrigation water adjustments

As you shift from spring to summer crops in the high tunnel, make sure you're checking your irrigation water. High tunnel soils are vulnerable to rapid pH changes (usually the pH increases due to high alkalinity and the addition of soluble salts to the soil in the absence of rainwater). Irrigation water that is more basic than the ideal 6.3-6.5 range can exacerbate the problem, resulting in soils that are not conducive to nutrient uptake. Growers can do a quick check with pH strips. If your irrigation water has a pH higher than 7, get your water tested for pH and alkalinity in a lab (like the UMN soil lab, $10 water test). You can reduce the pH of your irrigation water using acids like citric, sulfuric, or phosphoric acid, but you need to know the alkalinity in order to know how much acid to add to your water to bring the pH down to the ideal range. Alkalinity refers to the amount of bicarbonates and carbonates in your water. Read more about adjusting the pH of irrigation water here. 

Herbicide drift 

Mid may is the point in the season where we start to see herbicide drift cases. Take preventative measures to protect your farm: these can include signing up for DriftWatch, and talking to your neighbors and calling your local co-ops to make sure they know about your farm. 

Herbicide damage symptoms often present as puckered or curled leaves. Photo: Jed Colquhoun, University of Wisconsin,

Irrigation management

Most of you have likely been irrigating non-stop for the last couple of weeks. With meteorologists predicting a very hot summer, it's important for growers to plan ahead to make sure that you'll be able to provide sufficient irrigation to your crops throughout the summer, especially during critical periods like flowering and fruiting. If you don't already have a soil moisture monitoring system in place, consider investing in one now. This page provides an overview of options for soil moisture monitoring. 

Educational opportunities

The Vegetable Beet: join us during your lunch break on Wednesdays for a 30 minute discussion about vegetables. Next week's topic (May 26th, 11:30am) will be about summer lettuce production.

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