How can you tell whether a plant is hardy or not? Although frosty winter mornings can be beautiful, it’s heartbreaking to see your favourite plants fall victim to icy temperatures. But once you understand how plant hardiness ratings work, you can give your plants the protection they need to see them safely through the winter months.
What Does Hardiness Mean?
Hardiness measures a plant’s ability to stand cold winter temperatures in the gardening world. The United States Department of Agriculture has developed a hardiness rating system that classifies plants in accordance with the minimum temperature they can withstand. This rating system runs from hardiness level 1 (plants that can survive temperatures below -45.6 °C/-50 °F) to 13 (plants that must be kept above 15 °C/59 °F).
Not all plant websites or labels classify their plants using the USDA ratings, so it’s helpful to know some other terminology used to describe hardiness and how it relates to the RHS definitions.
- Tender – these plants can’t survive temperatures below 0 °C/32 °F and need to be grown indoors in winter, although they may be placed outside in summer.
- Half-hardy – these plants will typically cope with brief periods of temperatures down to -5 °C/23 °F. They can survive winter outdoors in mild areas but will need protection in colder regions or hard winters. Half-hardy plants would be considered zone 4
- Fully hardy – these plants will survive a minimum temperature of at least -40 °C/14 °F. They correspond to Zone 3
NB: The hardiness of a plant indicates the minimum temperature that it can survive, but other factors also need to be considered. One of the most important is soil drainage. A hardy plant can still be killed by cold weather if it’s waterlogged, and a borderline hardy plant is more likely to survive a cold snap in well-drained soil.
How to Protect Plants in Winter
Many plants will need some protection to get them safely through the cold months. Here are a few tips on how to protect your plants in winter.
- Here in Manitoba most plants will not survive over winter in pots above ground, as the roots of the plants will freeze.
- Move tender plants indoors into a greenhouse or conservatory to protect them from frosts.
- Tender perennials may be protected with a layer of straw, or extra snow, and this will increase the chances that an otherwise non-hardy plant may survive our cold winter
If you’re looking for plants that can cope with winter weather, visit our centre to see our superb range of indoor plants. Our staff will be happy to help you!