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Frost Protection

Going.Local
Published on May 3rd 2021
2
frost protection
Whip out those mittens, because it is going to be a cold one! While we cosy up to the fire, our plants seem to wither in the cold air. What can we do to help them slumber with ease this winter?

Do I need frost protection?

On the whole, you might find that areas along the coast and into the interior of KwaZulu-Natal remain largely frost-free. This is unless you take into account the recent uptick in snow forecasts for the lower-lying Cape Fold mountains. In the rest of the country, including areas of the Free State, Northern Cape, Gauteng, the temperature frequently dips below freezing.
Cold or Frost Damage: Cold damage will appear in the form of a burn or discolouration of the leaves closest to windows. Succulents may wilt or flop over as the frozen cells defrost and the tissue starts to decompose from the inside out.
A close up image of an agapanthus plant with frost damage

Frost

If you find yourself in a frost-prone area, it will benefit you to learn how to do some passive/active frost protection in the garden. Note that many tropical species will not do well when the temperature minimum drops below 10C, so this is equally important when you have acquired some recent houseplants.
frozen lawn
Photo by Rante Janne CC0 1.0

Passive protection

This is the first option and less costly. It involves several considerations to keep in mind when selecting a location for a plant.
Site selection:
  • Depressions (lower areas) in a garden or landscape will be colder and more prone to frost. Cold air is denser and will settle in the lower areas. These areas will need direct sunlight to heat up quicker in winter.
  • Soil type: Different types of soil will store and conduct heat differently. You can therefore suffer more damage when the soil cannot hold heat. Heat transfer from highest to lowest: Sand>Clay>Peat. In other words, organic soils we often use for gardening does not transfer heat well.
  • Place sensitive plants away from windows and windowsills. The air closest to the glass will be the coldest.
Planting:
  • The position of a tree or shrub will influence airflow and in turn heat transfer and radiation. Professionals often use smoke to study the flow of air in a landscape on frosty nights. This allows them to disrupt or divert the flow to avoid severe frost damage.
  • Some may opt to level out depressions, while others select more frost hardy plants for the lower-lying areas.
  • Choosing trees and shrubs with a delayed flowering time will avoid bud drop at the start of spring (or when there is a “false spring” followed by cold weather).
  • Canopy trees can “hold onto heat” or, more accurately, create downward radiation. An example is the use of shade trees in Brazil to protect coffee plantations.
  • Grouping plants can raise local heat radiation and minimise frost damage.
Snowy landscape
Photo by mac.black CC0 1.0
Plant Care:
  • Frost damage is more prevalent in unhealthy trees that have not received adequate fertiliser. They lose leaves quicker and bloom too early. Fertilising with the correct fertiliser makes the plants more robust.
  • Pests damage the protective layer that plants rely on during cold spells. Damaged bark or leaves will therefore die off quicker. Make sure to check plants regularly during late summer and autumn.
  • Pruning is subject to different plants, but it is good to prune grapevines later and citrus well before the first frost. In this case, it is good to keep in mind that pruning causes an open wound that should heal in time for the first frost.
  • Soil tilling or disruption creates air pockets that are poor heat conductors. This will add an additional chill factor.
  • Watering should be avoided in the days leading up to a cold spell. Rather water a few days ahead, allowing the sun to dry the topsoil but still contain some moisture.
frozen plants
Photo by Ranta Janne CC0 1.0

Active frost protection

Active management is a less cost-effective alternative reserved for prized or sensitive plants. It can protect plants that cannot be moved and do not mind the increase in humidity.
Here are some options:
  • Frost protection fabric: This is woven polypropylene fabric rolls, not to be confused with shade protection (see article on Shade & Greenhouses). They are normally labelled as 17 or 30g per square metre. The heavier fabric provides more protection (layering the fabric will increase frost protection). Note that each layer will block out more sunlight.
  • Plastic covering: Plastic is non-breathable, providing a way to capture and hold solar radiation. As moisture evaporates, it condenses into droplets on the plastic that should not touch plants (if the plastic touches the plant it will still suffer frost damage).
  • Cold frames: Cold frames work in a similar way to plastic covering, keeping the solar heat inside. It is best to build cold frames to face south, taking care that it does not fall in any shade during the day.
  • Extra insulation: A cold frame can be covered with additional fabric or bubble wrap to hold onto extra heat, but at the loss of filtered light.
  • Heating coils/mats: Commercial heating mats or coils are available for use indoors. These can be a great resource if treated with care (either put on a timer or alarm). Electrical appliances should be inspected regularly for potential wear and tear. Even if the appliance is waterproof, one should take precautionary measures to expose them to as little water as possible.
Some examples from the Candide community:

Addressing Frost Damage

A plant that has suffered frost damage is similar to one that suffers sunburn. The section of plant tissue that is frozen is essentially dead (there are a few exceptions where plants have evolved to produce a sort of antifreeze, but these are limited). The dead tissue will either dry out or (if it bursts open) slowly decompose from bacterial and fungal infection (i.e. rotting).
Removing tissue during the winter months is tricky and should be done as a last resort. Cutting or breaking off a piece of the plant will open it up to diseases. One option is to leave the dead tissue to dry out on its own and remove it come spring. The second option, if the discoloured area is spreading, is to remove the dead tissue and follow the procedure in the article below.
Remember that every frosted plant is a learning opportunity. It tells you which spot is too cold or colder than the rest of your space. This will help you select the appropriate plant in the future.

Any sensitive plants you need to coddle over winter? A precious few you cannot live without? Remember to tell me in the comments.

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