Protecting trees and shrubs from frost damage

“Tried and true” method of covering plants with lightweight fabric is usually the best protection during frost advisories.

What a difference a year makes. This time last year (as of April 11, 2013) our growing degree day accumulations were nearly a month ahead of normal and we had already experienced temperatures in the 80s, with more than a week straight of over 70 degrees Fahrenheit temperatures. This year, of course, is a different story, but spring will come and will likely come quickly when it gets here.

As trees and shrubs begin to leaf out and homeowners get antsy and begin to plant annuals, we need to be prepared for late frosts. Searching on the Internet for “plant frost protection” will yield a wide array of strategies for reducing frost damage. Some strategies such as frost irrigation or wind turbines are mainly geared to commercial horticultural operations such as orchards or nurseries. Other techniques such as a various spray-on products usually provide only a few degrees of protection or are variable in their effectiveness. For homeowners, the most effective technique is the old “tried and true:” covering plants loosely with a bed sheet or similar lightweight fabric. 

When covering plants for frost protection, Michigan State University Extension says it is important to remember the basic principle at work here. Late frosts typically occur on clear nights. That’s because the lack of cloud cover allows heat from the earth to re-radiate into outer space. By draping a sheet or other lightweight covering over plants, the radiant heat from the ground is trapped, preventing plants from freezing.

I bring this up because I have seen several forms of “plant protection bags” currently on the market. Some of these bags are designed to gather at the base, sort of like putting on an overcoat. This is another example where making an analogy between human function and plant function falls apart. Remember, the point of covering plants is to trap the earth’s heat, not the plant’s heat. Sheets or other lightweight fabric are a better choice than plastic since plastic will transfer more heat, and leaves may freeze when they come into contact with the covering.

More importantly, frost cover protection needs to be removed each morning as soon as temperatures begin to warm. Late frosts usually occur on clear nights, which mean the next morning is typically bright and sunny. Under direct sun, temperatures under frost covers can build quickly, resulting in heat damage to new growth.

Yes, going out to drape sheets over plants each evening and then removing them the next morning is a hassle, but like so many things in life, the tried and true is the safest bet.

Related MSU Extension article

Dr. Cregg’s work is funded in part by MSU’s AgBioResearch.