Updated: Nov 18, 2020
First lets explain plant hardiness. It refers to a plant's ability to survive unfavourable growing conditions. Plants are categorized into 4 groups based on their ability to withstand elements, particularly cold temperatures.
Plant Hardiness in the Vegetable Garden
Vegetable plant cold hardiness outlines the minimum temperatures that a plant can endure and still continue growing
When the growing season starting winding down, the days become shorter and the nights become cooler, tender and very tender plants need to be protected
Suitable means of protection include: floating row covers, plant blankets and plastic tunnels
These are known as season extenders. Plants are able to produce longer into the fall season because they are being protected from cold temperatures and possible damage from either a light or heavy frost
There is nothing that we can do to prevent frosts and freezes from happening, but we can reduce the damage that they do to our plants by being prepared
Watering plants when the temperature starts to drop
Watering plants before cold weather hits
Water can insulate plants. Wet soil holds four times more heat than dry soil
If you want to extend your growing season and avoid your plants getting hurt in frost we highly recommend a poly tunnel. You can learn more about them on our poly tunnel post!
Not ready for a poly tunnel? Here are a few other options to help avoid the frost.
What is Frost?
Frost is a thin layer of ice that forms on solid surfaces
Water vapour in above freezing temperatures from the air comes into contact with surfaces that are below freezing; ice crystals are formed
Frost's events are usually short-lived. They occur overnight or in the early morning hours
When frost occurs, it can be classified into two categories
Usually, occur when temperatures are between 0o C and -2 and the ground is not frozen
Light freeze/frost can kill/damage tender plants
Happens at the beginning and end of the growing season
This occurs when the temperature drops below -2o C. The air is cold and the ground is hard when hard frosts occur.
Usually, hard frosts make the beginning of the end of a growing season
Frost advisories are issued when conditions are favourable for the occurrence of frost during the growing reason. This is why there are hardiness zones with first and last expected frost dates
What is a Freeze?
A freeze happens when surface air temperature drops are 0o C or below
Frost may or may not occur
A light freeze occurs when the temperature is between 0o C and -2o C
Tender plants can be seriously injured if not killed
A hard freeze occurs when the temperature is between -2o C and -4o C
Can harm many plants in the garden, any tender plants will not survive a hard freeze
A severe freeze, often called a killing freeze happens when temperatures fall below -4o C
Extensive damage to plants occurs
Many plants can survive a short freeze but there are very few plants that can survive a severe freeze
*The main difference between a frost and a freeze is that a frost is visible, you can see it on the ground or on plants, and a freeze is when the air temperature falls below freezing. Frost is possible while temperatures are above freezing and sometimes we get a freeze without there being a frost. Frost is dependent on the amount of water in the air.*
Plant groups: Categorized by how well a plant holds up to frost and the temperatures that accompany it
Frost Tender (very tender plants): is killed after a few hours of being exposed to temperatures of 0o C to -1o C. Leaves, stems and buds will turn black from frost exposure
Semi-Frost Tender (tender plants): will have their leaves and stems injured if they are exposed to temperatures between 0o C and -1o C for a few hours. New growth such as buds and shoots will usually survive the cold
Semi-frost hardy (half-hardy plants): tolerate temperatures as low as 0o C to -2o C for short periods of time. The top growth of plants may suffer injury
Frost hardy (Hardy plants): can withstand a few degrees of frost. Temperatures in the -2o C to -4o C range as long as they are not sudden changes
*Above info is taken from the Ontario Ministry of Agriculture, Food and Rural Affairs website
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