Weekly vegetable update - August 27, 2025

Natalie Hoidal & Marissa Schuh

While the calendar says it is the end of August, it feels like late fall. But don’t be fooled! Warmer weather is on its way, and a long fall is likely. 

Taking advantage of a long fall

The NOAA predictions for September came out last week, and Minnesota is leaning towards having a warmer than average September. There may still be frosts at typical times, so investing in frost protection can help make the most of a potentially warm September.  

A warm September ahead of us? 

Methods for frost protection on sensitive crops include: 

• Row cover, with or without hoops: frost blankets can add a few degrees of protection to get your crops through a light to medium frost. The downside is that they are awkward to store - it’s important to dry them out well, keep them clean, and store them in a place where they won’t be eaten by mice over the winter.
• Irrigation. It sounds counterintuitive, but irrigating your crops during a light frost can actually keep them from freezing. Heat is released when water turns to ice, and so irrigating heavily so that there is standing water on your crops through the night offers a layer of protection. This isn’t meant to be a solution you use regularly. Rather, it’s a good fit for an early September frost when you just need to make it through the night, and when warmer temperatures are in the forecast. For this to be effective:
• Start when temperatures are above freezing, and keep irrigating until temperatures climb back up above freezing.
• Keep the sprinklers going all night, and make sure you have good coverage.
• Use as much water pressure as possible for good coverage.
• Check out the video below to learn about how Pine Tree Orchard uses irrigation for frost protection on strawberries (this section starts at 2:01):

Fall cover crop reminders

Now is a great time to plant cover crops. As your cucumber or zucchini beds wind down, you can plant cover crops one bed at a time to keep the soil covered across your farm and to fix some nitrogen for next year’s crop. Some of our favorite fall cover crops include:

• Early to mid September: Oats and peas are really perfect for this window. Planting in early September gives them some time to establish, outcompete weeds, and add some nitrogen to your soil. They will die over the winter and the residue usually breaks down nicely by spring. This is a great fit for fields you want to plant early next spring with cold tolerant crops like lettuce, spinach, and Brassicas.
• Late September through late October: Your squash, carrots, and other fall crops may still have another month to go. If you’re clearing fields in October, a winter hardy mix like winter rye or winter wheat + hairy vetch will keep your soil covered through the spring, prevent erosion, and the vetch can add nitrogen to your plots. These cover crops are best suited to fields where you plan to plant a later crop like tomatoes or squash next year, as they need some time to be killed and to break down in the spring.
• If you’re growing in a high tunnel and looking for a way to improve your soil health and get some nitrogen into the soil, it’s not too late to sign up for our cover crop trial! We’ll send you free seed (peas, vetch, clover, or all three), and we just ask that you fill out a couple of short surveys telling us how it goes. The fall planting window runs now through November 1. We also have a spring window for folks who are growing fall cash crops in their high tunnels. If you’d like to participate, reach out to Natalie at hoida016@umn.edu.

Oat and pea cover crop. Image: Adria Fernandez

Storage season

With fall just around the corner, the season for storing vegetables is here. Take a few minutes to review our postharvest handling webpage for reminders about optimal storage temperatures and humidity ranges for common vegetable crops.

Crop updates

Brussels sprouts

The next 10 days look balmy across Minnesota, with nighttime lows close to 60 in the south, and 50 in the north. However, mid September often brings cooler nights, and we often see our first hard freeze. As fall rolls in, growers often wonder about topping their plants. Brussels sprouts are one very common crop that gets topped. The idea of topping plants is to remove the apical meristem(s) (growing points) of the plant, which encourages the plant to shift its resources into ripening its existing fruit / buds vs. investing in new vegetative growth. Does it work?

Brussels sprouts tend to be sweeter if they are harvested after a frost, but they are also susceptible to freeze damage when temperatures reach 14 degrees or so. This makes the ideal window for harvest quite short. A study at University of New Hampshire found that while there were differences across Brussels Sprout varieties, early and midseason cultivars showed increase yields and resulted in higher quality sprouts if they were topped in early September. Topping also reduced the incidence of very small or very large sprouts, resulting in a more uniform harvest. However, if the sprouts had not yet reached marketable size towards the end of the growing season, topping did not effect yields. 

 

Image: Brussels sprouts that were not topped (top) vs. topped (bottom) from the UNH trial. HortTechnology 33, 2; 10.21273/HORTTECH05170-22

Carrots

We’ve had some of the wettest summer months on record, hence why we’ve written so much about disease this year. We took a look at some carrots in a UMN plot in Waseca with some sort of root rot, likely brought on by the wet conditions. When comparing this year’s rainfall over the last century, the station has experienced 10th wettest June, 5th wettest July, and 3rd wettest August on record this year. Carrots don’t love to sit in saturated soils, but pathogens do.

Diagnosing root problems is always tricky, and if carrots are a major crop, we highly recommend getting a sample to the plant disease clinic. In this case, we suspect bacterial soft rot because…

• The lack of any black or white spots, fuzz, or spores, which many other carrot root rots do cause
• The absence of a terrible smell, many other pathogens (and secondary infections) smell bad
• This year’s extreme moisture, as carrot soft rot pathogens are naturally present in soils, but only become a problem when things are very wet

Mushy carrots with suspected soft rot. Photo: Charlie Rohwer, UMN.

Cucumbers

Cucumbers are slowing down, it is cold, and the growing season is reaching its end.

Basil

Some basil plantings are at the end of their life, and basil downy mildew is being found. There have been major developments in resistant varieties, try some out next year if you haven’t yet.

Basil downy mildew causes dense, black fuzz on leaves and plant death. Photo: Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org

We did a little deep dive on basil postharvest handling this week: 

• Basil is quite sensitive to cold temperatures. Ideally, you could harvest and get it to your customers the same day.
• If you’re harvesting in small quantities, the best way to keep it looking nice is to harvest it in bunches and store it in a jar of water at room temperature. Tell your customers to keep it out of the fridge, where it can quickly become cold stressed. It tends to turn brown after returning to room temperature if it’s been exposed to temperatures below 54 degrees F.
• If you have to store it overnight, try to store it between 55-59 degrees F.
• High humidity helps to maintain freshness. Consider storing in perforated plastic bags. This holds in some humidity, while allowing CO2 to escape (basil is very sensitive to CO2.
• To prevent bruising (which can also cause brown / purple blotches), try to handle it as little as possible. This could mean harvesting it directly into the tote where you plan to store it.
• Try keeping the lights on in storage. Studies have shown that basil is sensitive to darkness, and that keeping the lights on in storage improves shelf life.
• IF you are washing your basil, use a sanitizer. Keep in mind that washing is another handling step that can easily lead to bruising of basil’s very tender leaves.
• While we usually encourage people to harvest in the morning when crops are nice and cool, there are some studies suggesting that basil should be harvested in the early evening for extended shelf life.

Tomatoes

On a lot of farms, tomatoes are slowing down. It has been a tough year for tomatoes, disease pressure has been high. Don’t rush to assume your tomatoes are just at the end of their natural life, we had a lot of foliar disease this year, and on a lot of farms, this is a major factor in tomato plants crashing out. For example, at a UMN tomato research plot in Waseca, plants are approaching a 100% incidence of early blight. It is worth the time to diagnose the foliar diseases you have, as you may be able to prevent the disease with a resistant variety or slow it down with a change in cultural practices next year.

Since we talked about topping Brussels sprouts, we might as well discuss topping tomatoes too. This is a common practice among growers, specifically for indeterminate tomatoes. There is limited research on how much this actually helps, and we could debate the value of your time and energy vs. the value of a few more tomatoes. Growers typically top their tomatoes ~4 weeks before the final harvest, so now would be the time to do it for outdoor tomatoes. Even for high tunnel tomatoes, plants really start to slow down in late September, with green tomatoes taking forever to ripen if they ripen at all. Instead of topping your tomatoes and waiting, some growers prefer to just harvest everything, including green tomatoes towards the end of September. Green tomatoes will very very slowly ripen (and sometimes don’t taste amazing when they do), and the cleared out high tunnel can be used to plant lettuce or a fall cover crop. 

Zucchini 

Powdery mildew continues to develop, and more and more plantings are reaching their end. Damage from squash vine borer is evident in some spots, though we didn’t hear as much about this bug as we do many years.