Wednesday, December 7, 2016

Cold Temperatures Pose a Risk to Kentucky Strawberries

Chris Smigell, Extension Associate for Small Fruits/Vegetables, 
University of Kentucky  

A cold front forecast for Thursday night may bring temperatures down to 18-19 ºF for all of Kentucky.  Strawberry plants are not fully dormant by now, so it is important to have some frost protection in place.  

We recommend  applying straw over matted row strawberries when the air temperature first drops into the low 20ºs F.  Mulching actually has several benefits:

  • Protects the plant roots from frost heaving. Cycles of freeze and thaw, lift the soil and plants.  This can break roots off,  especially the very smallest root hairs, that are critical for nutrient uptake.  This reduces berry size and yields.
  • Protects the plant crowns from winter injury. Fully dormant buds in crowns are hardy down to about 10º F, but it takes several freezes and consistent cold to fully harden the plants.
  • Minimizes plant desiccation due to winter winds.
  • Minimizes risk of black root rot  

A standard  straw bale should cover about 50 square feet of bed. A 3-inch layer of straw on an acre requires 3 tons of straw.

Leaves are not recommended. They tend to mat down which  can cause rotting and limit light penetration in the spring.

Frost heaving is less common on coarsely-grained soils. Clay-based soils are finely grained.   Any practice that helps drain the soil, such as adding organic matter, trenching, or tiling, helps reduce the risk of heaving.

It can take temperatures much lower than 32  to freeze all of the water in soil, depending on soil type. Thus  even an inch of straw can help reduce the percentage of water frozen in a soil profile, and the less freezing, the less the amount of heave. The following article describes the frost heave process. 

Here's a guide to correctly applying mulch. (photos by John Strang)




Applying plastic mulch in plasticulture systems is a multi-person job, so line up a few helpers, and set out mulch and weights to hold down the sheets ahead of time. Rock bags tend to work better than sod staples or cinder blocks which can tear the fabric. Plasticulture growers that are using lighter weight covers (.5-.75 oz/square yard) could apply one layer now to help protect the plants and then follow up with a second application later in the month or early next year when the extended bitter cold temperatures usually begin.

"Why not just put down straw or apply row covers well ahead of any critically low temperatures?"   Covering plants too early (late October, early November) can prevent late plant growth on those sunny days when temperatures are in the higher 50's. Second, plants need to be in full dormancy once winter really sets in. It takes several freeze cycles and cold soil  to bring plants into full dormancy.  This process will be delayed by applying mulch or cover too soon.  Generally by late November, it is safe to do so. Plant color is another guideline - strawberry leaves should have a grey-green appearance, indicating onset of dormancy.