Frost Damage

To keep with the Halloween theme, here’s Arlena’s image of her succulent stuffed pumpkin.

There are a couple of pointers to remind me of the oncoming cooler weather. Two being the shorter days and the ever decreasing times that I can take my dogs for a walk in the daylight around the park. Then there’s the butter dish on the kitchen table. Gone are the days when I can scrape out big dollops of lovely creamy butter onto my knife. It’s now a case of placing lumps on the toast and letting it soften. We are slowly being acclimatised to the colder weather so when the first frost comes it’s not too much of a shock to the system. Plants also acclimatise themselves, not by putting the butter in the microwave, but by changing their metabolism, anatomy and physiology to prevent frost or chill damage.

How do plants cope?

One response plants have to dropping temperatures is the thickening of liquid within plant cell membranes, from a liquid to a gel-like state with amino acids. This slows everything down, conserves energy and lowers the freezing point, a bit like antifreeze. At the same time, the cell membranes themselves become more permeable, allowing water to leak between the cells as a kind of buffer. This is the process of cold acclimation.

Acclimatisation is also known as ‘superfreeze’ where the cell contents remain liquid even though below freezing point. To do this plants have to experience several days of cold weather before the freeze and this explains why even hardy plants can be damaged by a sudden autumn frost.

2 Types of Frost

The two main types of frost are advective or radiative, depending on the atmospheric conditions. An advective frost occurs when cold air sweeps in, usually with strong winds. A radiative frost occurs on clear nights where there is little or no wind.

  

Frost can Benefit Plants

Frost can actually benefit some plants. Deciduous fruit trees benefit from winter chilling, and cold snaps turn starches to sugar in crops such as parsnips, improving their flavour.

Frosts can also disrupt pest and disease cycles, and improve soil structure – when moisture within soil freezes, it expands, and splits open soil particles.

Prevention of damage

There are a number of ways to keep your plants safe during cold weather;

Choose plants that are reliably hardy and suited to your growing conditions. 

Select planting positions carefully to avoid ‘frost pockets’ this usually happens at the low point of the garden.

Slightly tender plants should be grown in a warm sunny spot, against a south-facing wall for example, which will provide some extra warmth and winter protection.

Cover plants with a double layer of horticultural fleece or other suitable protection when frost is forecast. This cover is thin enough to let sunlight in, warming the soil and to keep in the warmth at night as the temperature drops.

Mulch the root area of evergreens now, conifers, tender shrubs and tender perennials with a thick layer of organic matter to prevent the ground becoming frozen. This is better done in warmer weather as adding mulch later on could trap the cold air in the ground.

Move container-grown plants to a sheltered part of the garden in cold weather and provide some extra protection by wrapping the pot in bubble wrap.

Leave the previous seasons growth on more tender plants until spring, for example penstemon, as this provides valuable frost protection during the winter.

Tender plants can be lifted or moved to a more sheltered position or greenhouse. If this is not practical then protect them by wrapping them in sacking. 

Lift tender perennials such as dahlias, cannas, pelargoniums and fuchsias before the first frosts.

Hardiness can be improved by feeding plants high concentrations of soluble nutrients including potassium and sugars in the growing season. Some plants also have physical protection such as thick bark, furry leaves or a canopy that protects the inner shoots. Avoid applying nitrogen-rich fertilisers late in the season though as they stimulate soft, sappy growth which is especially vulnerable to frost damage.

Plants exposed to early morning sun may thaw too rapidly after a frost, causing damage to flowers and young growth. Camellia and magnolia flowers in particular can be ruined by a single frost.

Ensure tender plants are overwintered safely in the greenhouse by providing adequate heating or insulation

It's the late frosts that we should be the most wary of. Plants emerge gradually from dormancy in spring, but they can still respond quickly to sudden spikes in temperature which triggers new fresh growth which is very vulnerable to cold snaps. It’ll be a while before we need to give that any thought though!