Winterizing Strawberries

November 20, 2023

Extreme cold has a negative effect on strawberry crowns. It causes winter damage and brown discoloration inside the crown. This is a really big deal because it causes a “clogged filter” in the crown which prevents rapid uptake of nutrients and water to fill the berries next spring. Which means low yields.

The critical threshold is 18-20 degrees. Research indicates that once the inside of the crown reaches 15 degrees, it can mean fatal crown damage. This is why plants need to be winterized and protected. Protecting plants is the most practical way to prevent winter damage.

The best protection comes from snow. 6-8 inches of snow will keep plants sufficiently warm, even in below-zero temperatures. The challenge here in Pennsylvania is that we don’t get sufficient snow cover the whole winter, which means that we need another source of protection on most winters.

Straw mulches are the most common because they do a better job of insulating the plants from cold. Wheat straw is preferred because it is more resistant to matting than other straw, including rye or sudan grass straw. Oats and barley straw are not good options either as they tend to break down more quickly and thus provide much less insulating value.

Covering plants not only helps to keep temperatures more uniform during temperature fluctuations, but also aids in retaining the moisture essential to crown health, while also promoting optimal drainage. It also reduces “heaving” from the freeze and thaw action, which is hard on the crowns as well. Another benefit of straw is that it also holds snow and keeps it from drifting and melting away as fast, which provides additional insulation.

4-5 tons of straw per acre is generally recommended. To apply the straw, drop it around and over the plants by loose handfuls. Mulch 6-8 inches thick. You may want to re-apply after a week or two, as it tends to settle.

Row covers also work if used properly. If using 1.25 oz row cover, a good rule of thumb is to use 1 row cover at 20 degrees air temperature, 2 covers at 10 degrees, and 3 covers at 0 degrees.

Some have used a combination of straw and row cover – the straw underneath, then the row cover over top for additional insulation and to hold the straw in place. Row cover will generally push the plants for earlier bloom and harvest.

While the mulch or cover must be applied before it is cold enough to get winter damage, it is also important that it isn’t applied too soon – we want the plants to go dormant first. We want the leaves to flatten out and turn brown – both indicators that plants are entering dormancy. Ideally the plants are dormant before we get the first 20-degree cold snap. If they are not, you may want to protect them overnight as a temporary fix. Usually (weather dependent) they are mostly dormant by late November to mid-December. A good rule of thumb is to cover during this time period, once most of the foliage is brown and floppy, and before temperatures drop below 20°F. And especially so if you have had several 20-21 degree nights.

Dormancy can also be sped up by foliar feeding a heavy dose of boron, along with potassium. Both are sugar movers with boron focusing on movement to roots. We have not yet identified the “perfect” amount.

In summary, foliar feeding for dormancy and then covering at the right time increases winter protection.