How to understand a plant's hardiness data?

What is a hardy plant?

In botanical terms, the hardiness of a plant is essentially its ability to adapt to climate conditions different from those it had in its native range. As many plants come to us from all five continents, with temperatures spanning wide extremes. If they have adapted beyond their original range, plants will therefore usually have to withstand a range of sub-zero temperatures that will not threaten their survival. Hardiness is therefore always discussed relative to a given region, defined by climate zones. A Mexican orange tree, considered hardy in most regions of France, will not be hardy in northern Germany. But globally, a plant is considered hardy when it can survive a minimum temperature of -5°C.

An obvious example of non-hardy plants is the oleander, a Mediterranean plant, ill-suited to northern regions where the climate is not mild during the winter months, or an exotic plant such as the banana plant, which also risks frost if not planted in a region with mild winters.

→ Elizabeth tells you much more in Hardy plants and hardiness.

Woodbine, on the left, is hardy to -25°C. On the right, an Agave attenuata, or ‘swan-neck’ agave, is, for its part, not hardy, suffering from the first frost.

Two major criteria to understand hardiness

•  Hardy plants are predominantly deciduous plants, those whose foliage disappears completely in winter: they do not have to photosynthesize all year round nor fear very cold winters since they go dormant by shedding their leaves. Their ability to withstand the cold is therefore increased. A large number can normally be planted pretty much anywhere within our latitudes.
  Some examples : deciduous clematis, many perennials such as daylilies (Hemerocallis) or Veronicastrum, or roses. Conversely, evergreen Ceanothus, evergreen Agapanthus, Loropetalum or Gardenias, for example, are frost-tender and are listed in the category of “plants for mild climates”.

Pelargonium Heydichium, Gardenia, Hebe: they are frost-prone!

There are, of course, exceptions among evergreen plants, often of European origin, which do not fear our winters, since they are native to our regions. They can be found in the category of hardy evergreen shrubs : European gorse (Ulex), privet, Mahonia, Ilex sp. (holly), are good examples.

Deciduous or evergreen trees and shrubs, well hardy here: Hamamelis, Viburnum plicatum, ornamental Prunus or Mahonia

• When talking about hardiness and giving a minimum temperature tolerance a plant can endure, be vigilant! This always applies to mature plants or those at least three years old. So pay careful attention to the hardiness data listed on websites as well as in botanical literature or on labels when buying from a garden centre or nursery. When it says “down to -6°C”, that means the plant will withstand -6°C if, and only if, it meets the following conditions:
  — Planted in the ground.
  — Planted for at least three years and protected by a protective mulch.
  — Planted in well-drained soil, which does not become waterlogged in winter, a pitfall often fatal to plants, regardless of their hardiness.

Before these three years, we should therefore adopt the practices we are used to with frost-prone plants: mulching thick and, where appropriate, garden fleece from the first frosts.

Factors influencing plant hardiness

With that premise established, the question remains: to what extent is a plant hardy? There are factors that can vary a plant’s cold hardiness by several degrees, and we detail them a little further here:

• Soil type: it is always THE question to ask before buying a plant, especially if it sits near freezing temperatures for your region. Drainage is essential for a large majority of plants, particularly the less hardy ones. Sandy or loamy soils will protect plants by retaining some warmth, whereas clay-heavy soils will have a disastrous effect, accumulating moisture, and thus making the root system feel colder.
• Potted planting: buying a plant to place in a border or in a pot on a terrace or balcony is very different, and shifts hardiness data by several degrees. In a pot, a plant has little soil volume available to protect its root system from the cold. Second pitfall: the pot material itself can be detrimental to the plant, retaining more or less moisture. It is estimated that pot planting reduces the plant’s hardiness by 2–3 degrees.
• Planting in a sheltered zone: planting alongside a wall or beneath the canopy of trees are garden spots that act as a microclimate, lifting the coldest temperatures by 1–2 degrees. In this case, it is the reverse, the plant benefits from a few extra degrees that will be salutary, especially if it sits within what is called the hardiness limit for your region.
• Conversely, planting in a windy corridor or in a hollow: These zones increase the cold intensity with wind exposure (areas not sheltered by tall trees or hedges), and stagnating moisture for planting in a hollow. Result, plants suffer from a few degrees of cold that can be harmful and may die at the peak of winter.
•  The region and climate disruption: Negative temperatures are felt more acutely at altitude and in certain regions than in others, depending on humidity. Likewise, winds in France do not have all the same effects from north to south. We refer here to hardiness zones, but they remain only an indicator in light of the factors influencing plant behaviour discussed above. Even more worrying, climate disruption increases temperatures across the globe, certainly, but we have observed for several years now that it also intensifies the severity of frost periods in winter. It is therefore wise to think twice before buying exotic plants for regions that will suffer, even for a day in a year, exceptional temperature drops, capable of overnight wiping out a plant that had managed to grow well up to then…

→ Read also our articles: How can I make my soil more free-draining?,  Microclimates in the garden, The ABC of pot planting,

Here, a bougainvillea is planted in the ground, south of the Loire, against the south-facing wall of the dwelling: a particularly sheltered area.

A few tips

Tip 1: before buying plants at the hardiness limit in your region, observe your garden and its different zones over a year. This helps identify the most sheltered spots that will accommodate some of the more tender plants. You should pay particular attention to areas that stay frozen for long periods in winter. If possible, record the temperatures during an especially cold winter to determine the range of plants you might consider.

Tip 2: plants that perform well in a region are generally available from local nurseries. Don’t be tempted by unsuitable plants which, sooner or later, will fail without warning…

Tip 3: Don’t push your luck! … and if you really want to treat yourself, knowing that your region lies at the hardiness limit for a tree, shrub or perennial, give it every chance by planting this beauty, often exotic, only in spring!

Finally, at Promesse de fleurs, the Plantfit tool helps you better understand the characteristics of your different zones in the garden. A bed-by-bed zoning will help you place plants that may be well suited to each area.

A simple visit to the nearest nursery is a real time and money saver: identify the plants that will thrive in your region.