Protecting Plants During a Freeze
Stephen Brueggerhoff, CEA – Horticulture; Texas A&M AgriLife Extension Service
December’s cool weather is always welcome like an old friend that briefly visits for a few months, reminding that we have thick layers of fabric in reserve to ward off the bite of frost or sustained freezes from less-than fair weather guests. Understanding types of cool weather events is key to properly dressing up our plants in just the right fashion for adequate protection. I have adapted an article written by AgriLife Specialist Monte Nesbitt and Horticulture Agent Skip Richter to describe the difference between frosts and freezes, as well as materials to consider for cover to help us make sound choices for winter protection.
Frost, specifically radiation frost, is used to describe heat loss from radiant energy, accompanied by freezing temps forming ice crystals from dew collecting on surfaces. These events occur with clear skies, calm winds below 4 mph and temperature inversions. Freezes, also known as advective freezes, occur when freezing air mass displaces warmer air with wind speeds more than 4 mph, an event that can cause ice crystals to form and pierce cell walls within vegetative tissue. Thawing allows fluid to leak out of damaged cells and causing a burned look to leaf and stems. Our job prior to anticipated radiative events is to trap as much heat as possible from the surrounding environment, slowing down heat loss just enough to prevent damage to plant tissue and with appropriate materials such as porous fabric. Commercial frost cloth can be used, and additional materials can include bed sheets or surrounding the target plant with cardboard. Since the focus is slowing down heat loss, you can also use surrounding structures that reflect radiative energy such as roof overhang or even placing under the canopy of larger landscape plants to reduce cooling. The best method for covering plants is securing material at ground level and within the canopy drip line instead of tying around the trunk of the plant. Remember we are not trying to insulate the plants, rather slowing heat loss. The difference may be measured by a few degree’s, but it will be enough to lessen the effect.
Advective freezes require more than lightweight covers for protection and can include building box-shaped frames, applying row covers or hoop houses to place over your plants. Polyethylene, 4 mil sheeting is a good material to cover these structures. Make sure that there is enough headroom between the plastic and plants because plastic is conductive and can damage vegetation on contact. Always remove coverings as temps warm above freezing to avoid heat stress. Buildings and surrounding vegetation can also be useful as windbreaks, disrupting increased wind exposure during freeze events. Protecting plant crown and roots can be managed by 3 to 4-inch layer of mulch. Make sure the plants are well watered prior to these climatic events. Water and wet soils collect heat during the day and release slowly as ambient temps cool, providing additional protection to your plants. You can also collect water in used 1-gallon containers or 5-gallon buckets, allow to warm during the day and place beneath covered plants to provide further protection.
Be prepared and have the right kind of materials on hand, and always use best practices to protect your plants from frost and freezes. Browse online to our website for more practical gardening information: https://brazoria.agrilife.org/horticulture/. Season’s greetings to you and your family, I look forward to serving you soon with outstanding horticulture programs, and I will see you in the garden.