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Weekly vegetable update 5/24/2023

Authors: Natalie Hoidal, Emily Hansen, Quincy Sadowski

Fields are beginning to dry out, and the week ahead will be ideal for transplanting. This update includes reminders about early season insect pests and irrigation needs, information about harvesting from new rhubarb plantings, wildfire smoke safety best practices, and tips for keeping transplants healthy when planting is delayed.

Crop updates

Cole crops: We’ve seen some beautiful early spring cole crops nearing maturity in high tunnels, but field production has mostly been delayed due to wet fields. Cabbage maggots are flying and laying eggs, flea beetles are out, and we’re already seeing diamondback moth feeding. This means growers will need to be extra vigilant about keeping seedlings and small transplants protected from early season insect damage. Row cover is the best-bet for avoiding all three of these insect pests, which all have different types of lifecycles and different management strategies.

Diamondback moth larvae and pupae on cabbage near Hastings 5/24/23

Sweet corn: Soil temperatures are reaching 60-65 degrees fahrenheit, which is ideal for planting sweet corn. Seedcorn maggot is active in Minnesota, but planting into warm soil helps to mitigate problems.

Carrots and beets: Since we will not see rain this week, make sure you’re irrigating regularly to support carrot and beet germination. Many growers have had success using row cover or even plastic tarps to retain soil moisture until germination, at which point the row cover or plastic should be removed.

Garlic: While you’re likely prioritizing irrigation everywhere this week to help with germination and transplant shock, make sure to not skip your garlic this week. Late May is the time when bulb development begins, and consistent soil moisture is critical at this stage. Typically we recommend applying about half of your nitrogen to garlic in the early spring, particularly if you used faster-release sources of N like urea. At this point, it’s fairly late in the season, and applications of nitrogen could delay bulb development. To gauge whether it’s worth applying more fertilizer, check your plants for signs of nitrogen deficiency, namely older leaves turning yellow.

Nitrogen deficient onion leaves on the left. Photo: Ed Kurtz,

Asparagus: Asparagus harvest has been ongoing, and may be beginning to slow down based on observations in Central MN.

Potatoes: Potatoes have begun to emerge, which means growers should anticipate the emergence of potato beetles any day. In our 2020-2021 trials, we found that using row cover for a couple of weeks, followed by mechanical cultivation for weeds, then applying a thick layer of straw was helpful for reducing populations. These preventative methods will likely not be enough to prevent damage: growers should still scout regularly for young larvae and consider spraying when larvae are small.

Cucurbits: While we haven’t seen any cucumber beetles yet, they tend to emerge right as we begin planting cucurbits. If you’re looking for something to listen to this week, check out our Vegetable Beet Podcast episode: Ugh Bugs! Organic Cucumber Beetle Management for a review of management strategies (it's available on Apple Podcasts, Spotify, etc.).

Harvesting Rhubarb

Climate projections show an increased frequency of wet spring weather in the future. Some growers have started to prioritize crops that allow them to consistently have something to include in CSA boxes or on farmers market stands in early spring during years like this one where planting is delayed due to wet weather. Rhubarb is a great choice for this.

Calls about rhubarb have filled up my voicemail box this last week. Most of these calls are from growers with rhubarb in their first or second year and are inquiring about harvest length and amount for their plant. Here are some tips for harvesting newly planted rhubarb:
  • Rhubarb should not be harvested in its first year so that the plant can successfully establish itself.
  • Wait until the second season before harvesting.
  • Wait until the third season to harvest if you started from seed.
  • Start harvesting stalks when they have reached full length.
  • Each variety is different but stalks range from 12 - 24 in. long.
  • When harvesting, only remove half of the fully developed stalks from the plant.
  • Do not use a knife to cut the stalks from the plant.
  • Hold the stalks firmly with your hand, pull, and twist.
  • After harvest:
  • Trim the leaves from the stalk.
  • Let the plant keep its leaves so it can build up energy for the following year.
  • If you see a seed stalk emerge, make sure to cut it off immediately. If you let it go to seed, the plant is using energy unnecessarily.

Holding transplants

Because planting is so delayed this year, some of you may be holding transplants longer than usual. If your plants are becoming “leggy” or light stressed, there are three methods you can try for reducing plant stress and slowing growth:
  1. Simply bring them outside, or open up the greenhouse as much as possible to reduce temperatures.
  2. Beyond reducing the overall temperature, a commonly used strategy in the floriculture industry is the DIF method, in which you keep your greenhouse warmer at night than during the day, which limits stem elongation. One adaptation to the DIF method that may be easier for growers trying it for the first time is the “cool morning pulse”. With this approach, you would reduce the greenhouse temperature for 2-3 hours at dawn, bringing the temperature 5-10 degrees (F) lower than the nighttime temperature (Cox, 2007).
  3. Another method for reducing plant growth is to use physical agitation. Agitation should gently bend the plant stems; too much force can lead to breakage. When plants are agitated a couple of times each day using physical brushing, running a stick or pipe over the tops, adding fans, or even brushing your hands through your plants, they slow their growth and put energy towards reinforcing the stem and building up their waxy cuticle layer. Take care to only do this when plants are dry, and with clean hands or sanitized equipment to prevent disease spread. 
Stressed peppers in too-small pots

Keep yourself and your workers safe from wildfire smoke

This week has been smoky, with air quality alerts across the state. Make sure you’re taking precautions to preserve your immediate and long term health while working in smoky conditions. We can look to California for guidance, where the state has a “Protection from Wildfire Smoke” rule under CAL/OSHA General Industry Safety Orders in 2019 (Title 8, Section 5141.1) which includes the following rules for employers:
  • If the AQI reaches 151, it is recommended that outdoor workers use a respirator. N95 respirators are the minimum recommended respirators. N99, N100, R95, R99, R100, P95, P99, and P100 are all suitable respirators because they provide more protection than N95s. The use of respirators by employees is voluntary, but they should be provided respirators to use by employers. Bandanas and respirators that do not have the above labels (e.g. N95) are not sufficient.
  • At an AQI of 151, employers are encouraged to modify work schedules, reduce work intensity, and schedule additional rest periods.
  • If the AQI reaches or exceeds 500, work will either stop or respirator use will be required. (This is a legal requirement, but respirator use is recommended well before this point.)
  • Respirators should be discarded after 8 hours. They become filled with particles and lose efficacy over time, so they should be considered disposable. Some respirators such 0as half or full face reusable respirators have filters that should be changed out every 8 hours.

See the UC Davis Western Center for Agricultural Health and Safety’s Wildfire Resources for more information and printable posters.

While these regulations are California-specific, they are a good guide for best practices to follow in Minnesota.

Vegetable Weather Report

Risk of frost

There is a frost advisory in effect for Northeast Minnesota tonight. After that passes, it’s looking like the risk of frost will have passed or nearly passed for most of the state, with 10 degree forecasts showing nighttime temperatures well above freezing for the week to come. The DNR publishes summaries of annual normal temperatures from 1981-2010, which they use to estimate last spring frost dates (less than 10% risk of 32 degree weather).


Most of the state is unlikely to see rainfall for the next 5 days or so, with potential for minimal rain around the middle of next week. This rain-free period corresponds well with warm soil temperatures, resulting in a great week for planting.

Soil temperatures

Soil temperatures are beginning to exceed 60 degrees Fahrenheit in most of the state, meaning warm season crops can be transplanted and planted. While it’s certainly possible to plant before this point, warm soil helps plants emerge and grow more quickly, reducing the risk for root rots. The following tables provide a snapshot of soil temperatures across the state. For more detailed info, see:

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