Module 13: Hardiness Ratings

To help gardeners evaluate the hardiness of garden plants, the RHS has developed a system of hardiness ratings. Beginner gardeners should use hardiness ratings as a general guide to assist in the purchase of plants. This ensures that selected plants can survive in their garden conditions.

Lesson Objectives

  • Understand why hardiness ratings are important
  • Identify the different hardiness ratings

Why are there hardiness ratings?

As a beginner gardener, it’s important to understand the RHS hardiness ratings. These ratings provide information about the level of cold a plant can tolerate, which is important to know when selecting plants for your garden.

When selecting plants for your garden, be sure to choose ones that are suitable for your climate and hardiness zone. This will help ensure that your plants thrive and grow well.

It’s important to note that hardiness ratings are only a guide. Other factors such as soil type, exposure, and microclimate can also affect a plant’s ability to survive cold temperatures.

What are the hardiness ratings?

H1a

This hardiness rating is for plants, typically houseplants, that need to be grown in a heated glasshouse or tropical climate. Preferring a temperature that is warmer than 15ºC.

H1b

These plants like to be grown between 10 and 15ºC. They can be grown outdoors in the summer in a sheltered and sunny location. They often perform better as a houseplant or under glass all year round.

H1c

These can be grown outdoors in the summer when daytime temperatures are high enough to encourage growth at temperatures between 5 and 10ºC.

H2

Tolerant of low temperatures between 1 and 5ºC, but will not survive being frozen. They require growing in a glasshouse until frost risks have passed.

H3

These are hardy in coastal and mild areas, except in harsh winters and areas that are at risk of early frost. Best placed in an unheated glasshouse when there are risks of frost or snow. These can be grown between -5 and 1ºC.

H4

Hardy between -10 and -5ºC, these are hardy in average winters. They may suffer foliage damage and stem dieback in harsh winters in cold areas at high altitudes. Pot plants are also more vulnerable, so try to protect them over winter where possible.

H5

They can survive temperatures between -15 and -10ºC throughout most of the UK. However, they may not withstand open or exposed areas. Many evergreens may suffer foliage damage, and potted plants are also at risk of damage.

H6

These plants can withstand temperatures between -20 and -15ºC throughout most of the UK. But potted plants are still at risk of damage if not protected adequately.

H7

These plants can survive temperatures below -20ºC in the severest of winters.

Factors that affect plant hardiness ratings

Several factors can affect a plant’s hardiness rating, and beginner gardeners must be aware of them to ensure that the plants they select are suitable for their growing conditions. Here are some of the key factors:

  1. Soil type: The soil type a plant is grown in can also affect its hardness rating. Plants grown in well-draining soil with good nutrient levels are generally more resilient than those grown in poor soil.
  2. Exposure: The amount of sunlight, wind, and other environmental factors a plant is exposed to can also impact its hardiness. Plants grown in full sun may be more resistant to cold temperatures than those grown in shade, while plants exposed to strong winds may be more susceptible to damage.
  3. Microclimate: A microclimate refers to the unique climate conditions in a specific area, such as the temperature and moisture levels. These can vary even within a single garden and affect a plant’s survival ability in cold temperatures.
  4. Watering: Overwatering or underwatering a plant can also impact its hardiness. Overwatered plants may be more susceptible to root rot and other diseases, while underwatered plants may be more vulnerable to cold temperatures.

By considering these factors, you can better understand how plant hardiness ratings work and make more informed decisions about which plants to select for your garden. It’s important to choose plants that are well-suited to the specific growing conditions in your area to ensure that they thrive and grow well.

United Kingdom USDA Hardiness Zones

First introduced by the USDA (Department of Agriculture of the United States), hardiness zones aim to identify how well plants will withstand the cold winter temperatures within these zones. But remember that these maps are guidelines; they do not factor in any potential micro-climates, precipitation, humidity, summer heat tolerance, etc. Knowing your hardiness zone allows you to select plants suited to your area.

Diagram from Gardenia.net

The United Kingdom mostly sit in USDA Plant Hardiness Zones 6 to 9. This means that it enjoys cool winters and warm summers. The frost-free growing season is fairly long, with the last frost ranging from March to April and the first frost starting as early as October to the end of November.

What is a hardy plant?

When we talk about a plant being “hardy,” we’re basically saying that it’s tough and able to survive in a wide range of conditions. This can include things like extreme temperatures, drought, and disease. Hardy plants can adapt to different climates and soil types, which makes them a great choice for gardeners who want low-maintenance, easy-to-grow plants.

One of the great things about hardy plants is that they come in many different types, so you can find ones that will thrive in your particular growing conditions. For example, some hardy plants are conifers, which are evergreen trees and shrubs that can tolerate cold temperatures and poor soil. Other hardy plants are succulents, which store water in their leaves and stems and can survive in hot, dry climates.

Another type of hardy plant is the perennial flower. Perennials are plants that live for more than two years and often come back year after year without replanting. There are many different types of perennial flowers, each with its own preferred growing conditions. For example, some perennials like full sun and well-draining soil, while others prefer partial shade and moist soil.

Overall, choosing hardy plants is a great way to get started. These plants are generally easy to care for and can tolerate a wide range of conditions, which makes them a good choice. Just make sure to choose plants that are appropriate for your growing conditions, and follow the care instructions carefully to ensure your plants thrive!

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Updated on March 5, 2024

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