Understanding Plant Hardiness and Climate is a Key Gardening Skill.

We gardeners can never rest easy at this time of year, especially those of us who enjoy experimenting with some new introductions and borderline hardy plants. We’ve already had some sub zeros so far this year, and it’s likely we’ll have more. So this article is a brief overview of Hardiness to help you understand the many factors that affect plants during low winter temperatures. The key to anticipating how your garden will be affected is understanding how they can combine, rather than focusing on a single factor, like temperature.

Anticipation is crucial in seeing your plants through harsh conditions and we find that websites or mobile apps like Wundermap to be very useful because they forecast using data from local weather stations, rather than regions, and you can usually find one within a few miles of your location. We have also found over the years that Wundermap tends to be the most accurate, on the desktop version you can even see graphs of the previous night’s temperature lows.

The most important weather or climate conditions are as follows:

Microclimate.
Temperature and duration of low temperatures.
Wind.
Rainfall.
Snow and ice.

Microclimate.
This comes first because it influences all of the other conditions. Understanding the local geography of your location and how exposed or sheltered your garden is, or even different parts of it are crucial. Creating shelter such as windbreaks, woodland conditions, or planting near to buildings can make a huge difference to plants’ resilience during winter, even making a variance of a few degrees C in some cases.

Temperature.
Watching a forecast for low temperature is obviously important, but it is the number of days or nights that they persist for and how overnight lows combine with daytime temperatures that is critical. Many plants can withstand extreme lows for one night, below -10C, perhaps only sustaining superficial damage to foliage or new shoots, but several nights of -5, -6 or -7C can be far more damaging. In combination with daytime temperatures that are extremely cold, the effects of overnight lows are far more serious.

Wind.
We all remember the beast from the east (BFE) which was a freak combination of ground temperatures and continual easterly wind for 18 hours. This was an extreme example of the impact wind can have.The effects of low temperatures in combination with shorter durations of north or easterly winds can be bad news for plants too; easterly winds are by far the worst because they are usually dry and can desiccate foliage, but northerly winds can be bitterly cold too.

Evergreens are a particular concern when the ground, or soil in pots, freezes during windy weather. Wind will desiccate the foliage, and because the soil is frozen, the plant isn’t able to take up water through its roots to compensate. Where possible, a layer of fleece will provide some wind protection.

It’s good to remember though that plants are not affected by ‘wind chill factor’ – (when the temperature might be 0C but the wind chill factor makes it feel to us like -5C). Only warm blooded animals can sense how unpleasant it is, but to plants it’s still 0C.

Rainfall.
If low temperatures occur after or during a long spell of wet weather, the ground and the plants themselves will be saturated and much more prone to freezing at their roots and in their softer shoots or woody growth. We witnessed this last year, when a very wet autumn turned overnight to freezing. This can be a disaster for some fleshy plants, especially those like Agaves that hold water in their crowns.

Snow and ice
A light covering of snow is not necessarily bad news on its own; it can insulate plants from cold winds, and providing it doesn’t persist for too long, even do the same for soil temperatures. The main perils from snow cover arise from its weight, bending branches and foliage, even ruining fleece shelters you may have made over tender plants. If snow has gathered in the crowns of plants like palms or Cordylines and freezes enough to turn to ice it becomes more problematic, and before this happens it should be shaken out of the crowns of these plants.

Different kinds of plant hardiness.

Added to the mix of the weather conditions and their possible combinations, we need to consider how different species will respond to them. Rather than delving into each individual species’ hardiness, there are some general types of plant physiology that are worth understanding.

Foliage hardiness
Shoots, buds, and young stem hardiness
Woody growth hardness
Root hardiness

 

Foliage.

The leaves of plants are usually the first to sustain damage during the winter, and whilst the plant may look a little miserable it will usually re-foliate in warmer conditions and be in no danger of serious damage. Olives were a great example of this during the awful winter of 2009 when we had lows of -16C and ice on the ground for many weeks. Many lost their leaves and then re-grew their crowns back to their former splendour the following spring. A warm spring is the best antidote to this type of damage, encouraging new growth early in the season.

Shoots, Buds, and Young Stems.

If your plant suffers this type of damage to the tips or outer regions of its structure it can usually still be regarded as superficial; the plant will regenerate during the spring, growing from new buds and shoots lower down its branches or stems. A lot depends on the suddenness of low temperatures at the onset of winter, and how warm the late summer/autumn has been. A slow decline of temperatures into winter allows plants to harden off and go into dormancy. The previous winter, of 2021/22, was an example of a sudden winter onset after an unusually warm and wet autumn. After a drought and heatwave in the summer, during which time most plants stopped or slowed their growth, they suddenly flushed when wet and warm temperatures began in autumn, only for this tender new growth to be brutally damaged by sudden low temperatures. Callistemons in particular and seemingly all antipodean plants like Eucalyptus suffered from this. Emerging spears of new palm leaves, where they are at their most tender in the centre of the crown, suffered leaf burn, even those of the incredibly hardy Trachycarpus fortunei.

Woody Growth.

Extreme and persistent low temperatures during or after heavy rainfall can affect the tougher parts of plants and cause varying degrees of damage. Plants can still recover from this depending on how far into old wood the damage occurs. Newer woody growth may rot, and careful observation during the spring is required to monitor how far the wood has softened. Pruning is usually required to remove the decaying and damaged parts of stems before the rot spreads to firm growth. This is particularly the case with Cordylines as we found out at Beccles after the winter of 21/22, when we didn’t remove rotting growth from some young plants quickly enough. Bark splitting on shrubs and trees can be bad news, usually resulting in their slow decline, as with our magnificent Callistemon ‘Mauve Mist’ at Beccles, which seemed to sail through the 21/22 winter, unlike others in the genus. The split became apparent during the summer and it suffered some slow die-back during the summer. We can’t tell if it will recover just yet.  Plants may recover slowly, like our Eucalyptus gregsoniana which seem to have taken their bark splitting better than our Callistemons.

Roots.

Many plants can sustain a total loss of their growth above ground and will regenerate from woody or fleshy rhizomatous root systems. Cordylines and Eucalyptus are great examples of woody root regrowth. In the case of Cordylines, they are able to re-shoot from the base of the damaged trunk or from below ground and on occasion a small distance from it. Many Eucalyptus have a swollen root at their base called a lignotuber which can send up new growth after bushfires in the outback, and crucially from cold damage in European climates. Musa basjoo, the hardy banana, is an outstanding example of a root re-generator. At our Costessey branch, we thought they were surely dead after the winter of 2009 when they were left planted out and unprotected. Unsurprisingly the top growth quickly turned to mush, but they regrew to 2m tall plants the following summer. Other species such as Cannas and Gingers regenerate from rhizomes, thick fleshy spreading roots, and are easily protected during low temperatures by mulching with straw.

Hardiness Guide – not available via our website, exclusive content for our Newsletter subscribers

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