Maintaining healthy transplants through cool weather

Author: Natalie Hoidal, UMN Extension Educator for local foods and vegetable production

Spring temperatures have been cool across the state, and some of you may be holding onto your transplants longer than usual. In Minnesota this May, we’ve accumulated anywhere from 35 to 75 fewer growing degree days than what we would see in an average May. There are a few things you can do to slow down transplant growth prior to transplanting, and to reduce transplant shock:

Before transplanting

Limit growth if you need to hold transplants longer than expected

If your plants are starting to stretch and you’re not ready to transplant into the field, there are a few ways you can reduce growth and stem elongation. By lowering the overall temperature of your greenhouse or high tunnel, plant growth will slow down. This may also help to harden off your plants and reduce transplant shock. Another option is to simply bring your plants outside during the day if doing so is feasible with the scale of your operation and quantity of transplants.

Image: Scott Streble 

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).

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 sterilized equipment to prevent disease spread.

Finally, reducing your fertilization rate can also slow plant growth. 

Re-pot if needed

If your plants are becoming root bound, you may need to re-pot them into larger cells or containers (this is only really practical on a very small scale). If you are repotting, make sure to only do so with plants that are not sensitive to root disturbance. Tomatoes and peppers are quite hardy and can handle repotting; cucurbits are very sensitive and will not respond well to root disturbance. This is by far the most time intensive of the options previously discussed, and is really only necessary for very root-bound crops that you do not plan to transplant for a few weeks. A common practice is to start seeds in 128, 200, or 288 cell trays to economize space in a germination chamber, and repot into 50,72, or 98 cell trays. But, if you intend to leave them in small cell trays, time their seeding closer to your intended transplant date, and pay close attention to watering. MSU did a trial showing some performance differences in peppers that were linked to seed tray size. 

Image: UMN Extension

Hardening off

Many of the methods discussed above (reducing temperatures, bringing plants outside, reducing fertilizer) are essentially methods of hardening off. Hardening off is a process in which you condition plants grown in stable environments like greenhouses to survive in much more unpredictable field conditions. During this process, growth slows to allow plants to accumulate stored energy reserves, and to develop a thicker waxy cuticle on their leaves for protection.

This process is necessary for avoiding transplant shock, but over conditioning can significantly delay plant growth when you transplant into the field. Take care to avoid exposing your plants to very cold weather (50 for warm season crops, and below 45 or so for cool season crops), agitating them too much (more than a gentle agitation a couple of times per day), or withholding too much fertilizer or water.

For a more thorough discussion of hardening off and transplant health in general, see this excellent publication from Utah State Extension. 

When you’re ready to transplant

Create a warm environment

Slow-growing plants are more susceptible in the field to insects like cucumber beetle, flea beetle, and cabbage maggot, and to damping-off diseases. To facilitate rapid growth during cold weather, using plastic mulch or row cover will increase your soil temperature.
Consider soil moisture

If you have plants that are already rootbound or heavy clay soils, be especially thoughtful about when you transplant with regards to soil moisture. As a general rule, planting in wet conditions can lead to soil compaction. A root bound plant will struggle to expand its root systems in compact soils; if you have to plant in suboptimal conditions at all, prioritize planting more stressed plants during windows of optimal conditions. 

Consider staking or trellising

If your plants are stretched or “leggy”, they may need a little extra help in the field. Keep a close eye on plants as they establish, and consider staking higher value crops like peppers if you do not do so already.

Thanks to Anne Verhallen, Ajay Nair, Ben Phillips, and Matt Kleinhenz from the Great Lakes Vegetable Working Group for their excellent discussion of transplants and cool weather on the Great Lakes Vegetable Producer’s Network roundtable, which inspired this article. Tune in every Wednesday at 11:30am central time for a live discussion with Q&A, or listen to recorded discussions later.