Hardening Off: Or how not to kill your indoor seedlings

Have you ever gotten excited about the warm weather in May and gone and planted all your seedlings outside only to watch the leaves turn yellow or shrivel up?

This can happen when your plants get exposed to weather they aren’t used to. 

Let’s look at how to harden off your seedling  indoor-sown plants and prepare them to face the harsh reality of the spring climate in Quebec.

Why You Need To Harden Plants

You take great care to give your nursery seedling everything they need. You maintain the perfect temperature, adjust the light, and water them as needed. Not to mention there is no wind inside your house. Your little plant babies are very pampered.

Plants that are used to growing in these conditions do not develop the cellular structure strength to resist the direct sun, wind and cold of the outdoor world.

If you take these seedlings outside too quickly, the leaves can be sunburnt or damaged by the wind. They need to be gradually exposed to the outdoor weather. This is what is called hardening off.

Step-by-Step Hardening Off Guide

Ideally, you should take  7 to 14 days to harden your plant before planting them outdoors. 

Here is what that looks like:

After this two week program, you will be able to transplant your plants directly outside into the garden.

Outdoor Planting Tips

Here are some steps to make sure your seedlings do well when you finally plant them out after their hardening off period.

• Choose a cloudy day to minimize transplant shock as much as possible.

• Gently untangle the roots if the root ball is compacted. This allows the plant to recover faster and have better rooting. Be careful not to break them!

• Water immediately after transplanting. This reduces the stress of dry soil.

For frost sensitive crops (tomatoes, peppers, squash, basil, and many flowers), keep an eye on the forecast. If  there is going to a frost (often around the full moon), you will have to cover your plants to protect them.

How To Plant Frost-Sensitive Crops Earlier in Quebec?

Generally, you will want to plant your frost-sensitive crops only once there is no risk of frost. This will be from late May to early June depending on your hardiness zone.

If you are a slightly impatient gardener, you can use alow tunnel (or row cover) to plant earlier than your last frost date. This technique involves placing galvanized wire hoops over your garden beds then covering them with a floating row cover.

Choose Your Floating Row Cover

How early you can plant will depend on the material you use for your low tunnel.

Floating row covers are generally categorized by their weight in grams per square meter. The smaller the number, the lighter and thinner the cover is, the less it protects against heat, but the more it lets light and water pass through.

Here are the main options you can find at suppliers like Dubois Agrinovation:

• NovaGril P12: This is one of the thinnest covers. It lets through a maximum amount of light (about 90%) and offers very slight thermal protection. It is mainly used for protection against insects (carrot rust fly, flea beetles). 

• Agril P17 or Novagril P19: These covers are thicker and ideal for creating a small microclimate without the risk of overheating. They are very good in early spring. You can gain 2 to 3 extra degrees with this, which can help protect against light frosts overnight. The P19 is slightly more durable than the P17.

• BioPlus P30: If you plant very early and fear severe frosts, this cover offers superior thermal protection. However, it lets through less light.

The Polyethylene Plastic Film Option

It is also possible to use apolyethylene plastic film on the hoops as an alternative to floating row covers. Plastic allows you to achieve maximum warmth and can increase the temperature when it's sunny. However, you must be very careful about two things 

• Water Management: Unlike floating row covers, plastic does not allow water to pass through, which will require careful irrigation management.

• Ventilation: When it is sunny, the tunnel can overheat very quickly, risking burning your plants. Regular ventilation is crucial.

Now It’s Your Turn!

Now you have the tools to successfully harden your plants off and protect them from the weather. 

What will your hardening off strategy be? Where will you gradually expose your plants to the elements?

And don't forget to get what you need (row covers, hoops) to protect your plants in case of a late frost.

Cheers!

👨🌾 Frédéric