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Minnesota midsummer vegetable update

The middle of the summer in Minnesota is an exciting time for gardeners. We are harvesting the fruits of our labor, while still planting new things and tending to existing plantings.

While I love the abundance of greens and herbs in spring and early summer, I’ve been excited to eat cucumbers and zucchini straight from the garden these last two weeks, as well as currants, gooseberries, and raspberries. I’m looking forward to tomatoes and peppers soon! 

Common garden problems so far in 2020

A small spinach plant with a flower head (panicle) forming in the middle.
Spinach plant bolting

I spend my summers helping vegetable farmers troubleshoot problems in their fields, so I see a lot of diseases, insects, nutrient issues, and other random problems. So far, this year has been different from the last three years or so. Following three years of heavy and nearly constant rainfall (years full of diseases), we’ve had a very hot and dry summer.

Here are some things I’m seeing this summer: 

  • Fewer diseases, but more insects: We’re seeing high populations of cucumber beetles, Colorado potato beetles, aphids and leafhoppers (more info below), and caterpillars in brassicas.
  • More sunscald and windburn: Hot sun and dry winds can cause sunburn and windburn-like symptoms on plant leaves. 
  • Bolting: We had a very cold spring followed by a hot summer. These are perfect conditions to cause cool-season crops to “bolt”, or produce flowers when the plant is still immature. 

One interesting trend this year is the early presence of migratory insects. Usually, we see small, soft-bodied insects that fly in from the south around early to mid-July. These insects are transported on summer storms and wind currents from the south. However, Tropical Storm Cristobal in early June carried an abundance of aphids and leafhoppers.

These insects often carry diseases, and we’ve seen an uptick in viruses and leafhopper injury this year. These insects are harder to detect than larger insects that chew holes in the leaves of your plants; they tend to have piercing-sucking mouthparts, and so rather than chewing holes, they drain fluids from plants.

Plants with aphid and leafhopper damage tend to look stressed with yellowing leaves and often a puckered look, but the insects causing damage are tiny and often hide under the leaves, so the cause is not immediately apparent. 

Hand holding up an insect net full of small (around 1/8 inch), bright green winged insects.
Potato leafhoppers in a sweep net
Bean plant with puckered leaves.
Heirloom dry beans showing “hopperburn” symptoms

Problems to anticipate

Inconsistent water

When we have fluctuating levels of moisture in the soil, we start to see physical issues in plants.

  • Blossom end rot in tomatoes, peppers, and zucchini is caused by an inability to transport vital nutrients due to (mostly) inconsistent moisture.
  • Hollow heart in potatoes, and boron deficiencies in cauliflower and beets can also be exacerbated by inconsistent moisture.

Over the last couple of weeks, we’ve seen hot dry spells with sporadic but intense rainfall. Make sure you’re providing at least a little bit of moisture in those dry spells to prevent these problems.

A high-tech way to measure soil water content is to purchase a moisture meter. However, you can also just use your senses. Go out into your garden and dig around in the soil every couple of days. If it feels dry, make sure to water. 


This time of the year is when we really start to see diseases due to increasing humidity in the atmosphere. Scout your garden for diseases fairly regularly, and remove diseased tissues (such as the bottom leaves of your tomato plants). Do this with clean hands and tools so that you don’t spread diseases as you go. 

Branch of a tomato plant with brown spots and yellow halos around the spots, as well as some overall leaf yellowing.
Tomato plants are showing symptoms of typical diseases like early blight, Septoria, and Fusarium.

What can you still plant? 

While we are well into summer, there is still ample time to plant new successions. Maybe you have a garden bed where you just harvested lettuce and peas, and it’s now sitting empty. The Extension website has an excellent list of vegetables that can still be planted at midsummer.

Make sure to check for the first frost date in your area, and figure out the number of days between now and then. Choose a variety that fits that window. For example, some bush beans only need 45 days or so from planting to harvest, whereas others need 60 days. If you’re living in northern Minn. and you anticipate an earlier frost date, a quicker variety is a better choice. 

As always, Master Gardeners are available to help you troubleshoot problems in your garden.

Natalie Hoidal, Extension Educator, local foods and vegetable production

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