Author: Natalie Hoidal, UMN Extension educator, local foods and vegetable crops
What a difference a week can make! We went from freezing to unbearably hot temperatures, and plants have in many cases tripled in size in a single week. This next week will be a process of watering, weeding, and repeating. Many insects are beginning to show up, so make sure to set aside time for scouting as well.
Vegetable weather report
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| 7 day precipitation forecast, weather.gov |
We will continue to have hot weather for another day or two, and then temperatures are expected to drop to more reasonable levels. We can expect highs in the high 80's and low 90's, and nighttime lows in the 60's for the southern part of the state, and highs in the low 80's and nighttime lows in the 50's for the northern part of the state.
Most of the state is projected to receive less than an inch of rain in the coming 7 days, so irrigation will continue to be critical.
At this point soil temperatures have reached at least 65 degrees consistently across the state, so I will stop reporting weekly temps. You can always look up your soil temperatures at: https://app.gisdata.mn.gov/mda-soiltemp/
Crop updates
- Cole crops: Cole crops are looking great, and have sized up quite a bit this week. Growers are harvesting sprouting broccoli, bok choy, and kohlrabi, and radishes are looking great (even if they've got a lot of holes in the leaves)! Flea beetles continue to be a problem, and I saw my first adult imported cabbage worm moth this week. Start scouting regularly for cabbage caterpillars.
- Lettuce: I saw quite a bit of bolting lettuce this week - make sure all of your varieties at this point are specifically labeled as heat tolerant. Check out this conversation on the Vegetable Beet (scroll down to recent episodes) about summer lettuce production with Dr. Ajay Nair from Iowa State University and Emily Fagan from Humble Hands Harvest Farm for an in-depth discussion about good summer varieties.
- Potatoes: Potatoes are emerging and growing quickly. Some growers already have flowers on their potatoes, and for others, the potatoes are just emerging. Potato beetles have been out for a while now, but this is the first week that I saw first generation nymphs (the first development stage after hatching from eggs). If you're planning to use insecticides, particularly preventative ones like Bt, neem, and pythrethrins, this is the best time to do so, as they become more tolerant as they become more mature. If you're growing on a small scale and rely on physical removal, taking some time to squish eggs now can save you quite a bit of time later!
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| First generation potato beetles are laying eggs. Photo NH |
- Peas are flowering for many growers. While we are in a drought and disease pressure should be limited, the flowering phase is when we typically start to see more disease pressure.
- Carrots and beets are being harvested - for many this is the first week of carrots, but a few early high tunnel carrot folks have had them for a little while. Make sure you're shifting to more heat tolerant summer varieties as you plant new successions.
- Onions: Onions are around the 5 leaf stage at this point. Continue to keep an eye out for onion thrips as the weather stays warm and dry.
- Garlic: Garlic scape season has begun on a few farms in the far southern part of the state, but most people are still a couple of weeks out. Even though your plants may look more robust / less prone to wilting than some other warm season crops, maintain irrigation in this critical period of bulb formation.
- Asparagus: I'm seeing more and more asparagus at markets start to look thinner and smaller. If this is true of your asparagus planting, harvest all remaining spears (even the small ones), fertilize, and remove weeds, then wrap up harvest for the season. More info in the UMN asparagus guide. See more below about post-harvest irrigation.
- Cucurbits: I saw plenty of frost damaged cucurbits this week, but many were beginning to sprout new growth. Cucumber beetle populations are also climbing. See last week's update for more information about cucumber beetle management.
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| Zucchini plants that were damaged by frost putting on new, healthy growth. Photo NH. |
- Tomatoes and peppers: A few growers who started tomatoes very early in high tunnels are reaching the flowering stages. High heat can have an impact on fruit quality around a week or so before fruiting, so expect to see some inconsistencies in your first fruit. You may also also be seeing blossom drop if your plants have begun to flower. However, most growers are still solidly in the vegetative phase, and this heat wave should not significantly impact flowering or fruit quality.
Problems in the field and things to anticipate this week
Pruning in the heat?
Early June is an important time for pruning tomatoes, but we've received quite a few questions about whether it's ok to prune plants in very hot conditions. There is surprisingly little research on this topic, as most research related to high heat and tomatoes have to do with fruiting rather than early season development. That said, keep in mind the following:
- In one recent study, when tomato plants were exposed to ~96 degree weather during the day for 2 weeks and ~88 degree temperatures at night, they experienced enough stress to reduce root mass and root:shoot biomass ratios significantly. (Giri et al., 2017).
- Typically following pruning there is a shift in biomass allocation, in which plants utilize stored carbohydrates from the roots to stimulate aboveground biomass growth and recovery.
- If your tomato roots are already experiencing stress, pruning may trigger further stress as it depletes stored energy in the root system.
- Leaving the canopy a bit more dense than you typically would for the next couple of days may help to shade the soil, keeping the roots cool. Since we're in drought conditions and most farmers are not facing much disease pressure right now, there is little harm in leaving the canopy more dense than usual for a couple of days.
- All of that said, temperatures will come back to more normal levels in the week to come, so you should be able to resume normal pruning quite soon.
Bolting
Just as field sown lettuce, spinach, and Brassicas are coming into maturity, an extremely hot couple of weeks can trigger bolting. Keeping crops well watered can help to keep the soil cool, as can the use of light reflective mulches such as straw. These measures can help to prevent bolting. More info about bolting.
Should I keep watering after I finish harvesting my asparagus?
I came across a very useful paper this week in which researchers in Michigan tested whether continuing to irrigate asparagus during fern growth would result in higher yields in future years. They tested irrigating vs. not irrigating in two cultivars from 2012-2017: Jersey Supreme (a very commonly used cultivar that is no longer available) and Guelph Millennium (likely the most promising replacement cultivar at this point in time for MN growers). For Guelph Millennium, irrigation during hot, dry years resulted in yield increases in the following year. However, Jersey Supreme did not benefit from irrigation, and in one particularly wet year, irrigation actually resulted in lower yields the following year. You can read the full article here. We lack similar research for other common MN varieties such as Purple Passion and Mary Washington.
Educational opportunities
The Vegetable Beet: join us during your lunch break on Wednesdays for a 30 minute discussion about vegetables. Next week's topic will be about June bearing strawberries.
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 hoida016@umn.edu, and you can submit questions and requests for diagnostic help here.
Sources
Giri A, Heckathorn S, Mishra S, Krause C. Heat Stress Decreases Levels of Nutrient-Uptake and -Assimilation Proteins in Tomato Roots. Plants. 2017; 6(1):6. https://doi.org/10.3390/plants6010006
D.C. Brainard, B. Byl, Z.D. Hayden, D.C. Noyes, J. Bakker, B. Werling, Managing drought risk in a changing climate: Irrigation and cultivar impacts on Michigan asparagus, Agricultural Water Management. 2019; 213: 773-781.
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