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Smart Garden 2020: How to avoid stressing transplants

A basil plant experiencing transplant shock and sunscald. 
Photo: Annie Klodd 
With the warm weather ahead, you're likely feeling excited to get out into your garden and plant all of the seedlings you started indoors. Before doing so, there are a couple of key things to do to set your seedlings up for success.

Condition your seedlings for success

If you've been growing your seedlings in a warm window, they've had a pretty good life so far. Chances are, you maintain your home or apartment at a fairly stable temperature, and your plants have been relatively undisturbed. When you move them outside, they will suddenly be exposed to relatively intense temperature fluctuations, strong sunlight, gusting winds, insects, and potentially animals like mice or cats stepping or nibbling on them.  

When plants move from a well-protected indoor environment to your garden or field, they frequently experience "transplant shock". In order to prevent this, plants need to be "hardened off." 

Hardening off is a process of forcing your plants to invest in storing carbohydrates and back-up energy reserves, fortifying their stems, and developing a thick waxy cuticle layer over the leaves. All of these traits help plants to withstand unpredictable and harsh conditions. 

How to reduce plant stress

There are a few ways to accomplish this hardening off process. The first, and easiest, is to simply bring your plants outside each day. 
  • Place them in the shade where they won't get too much direct sunlight right away.
  • Leave them out for a progressively longer period of time each day so that they can experience cool early morning and evening temperatures. 
  • Just don't forget about them and leave them out if it's expected to freeze. 

If you have an unheated mudroom, shed, or garage, you could also leave them there for the night. This will allow them to get cool, without exposing them to the full cold of the outdoors.

Give your plants a 'brush'

Another approach is physical agitation. Brush your hands over your plants a couple of times each day to bend the stems. Don't brush too hard or the stems will break, and make sure you only do this when your plants are dry and your hands are freshly washed to prevent disease spread. 

Watering

Finally, watering less will also help to harden off your plants. If you've been watering every two days, cut back to every three. Keep an eye on your plants and water them if they're starting to look wilted.

All of these approaches will signal to your plants that they need to strengthen their stems, thicken their waxy cuticles over the leaves, and store energy. You do not need to do all of these things; too much stress can actually stunt growth or kill your plants. 

Wait until it's time to plant

For those of you who planted last week, you likely saw some damage in your gardens. Even cool season crops like broccoli struggle to make it through multiple hard freezes in a row, especially when the plants are young with underdeveloped root systems. 

Now that the cold front has passed, you should be able to transplant cool season crops like cole crops (kale, broccoli, etc.), lettuce, and peas. If you have not direct seeded already, these plants can all be direct seeded now.

For warm season crops, be patient a little longer. Warm season crops like tomatoes, peppers, and cucumbers do best when the soil is closer to 60 degrees. At the Iowa border, soil temperatures are hovering in the low 50s, and at the Canada border they are more like 45. 

While the risk of frost has passed for the Southern half of the state, Northern gardeners should be a bit more cautious. For detailed frost date data for your city or town, the DNR has an interactive map that tells you the likelihood of frost based on average temperatures

The screenshot below is from Redwood Falls, MN. After May 13, there's a 10% chance or less that the temperature will reach 32 degrees. 

Author: Natalie Hoidal, University of Minnesota Extension Educator, Local Foods and Vegetable Production



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