Hardening Off Seedlings

Pumpkin Seedlings

Hardening Off Seedlings before Transplanting Outdoors

The process of preparing plants for transplanting out into your garden Is called “Hardening Off Seedlings”.  Indoor transplants were born and raised in a calm, serene, temperature and humidity-controlled environment. The indoor environment usually has warmer temperatures, yet less than full sunlight. If moved directly outdoors, sudden weather and climate changes can lead to transplant shock. Importantly, the shock can slow the growth of your plant. It can even stunt the plant’s growth.

Gardeners often think of hardening off tomatoes and pepper plants. However, this process helps smooths the transition of any flower or vegetable plant into their new summer garden home.

Hardening off plants is the process of exposing an indoor-grown plant to the outside elements gradually, over some time. This allows it to get accustomed to widely fluctuating climate conditions. The hardening off process is usually performed over several days, by increasing the time outside each day.

How to Harden Off Seedlings for Transplanting Outdoors

Begin hardening off plants, by setting them out onto a warm, sunny patio or deck for just a few hours a day. Make sure to wait until the location has warmed up in the morning, before setting them out. Then, bring them back indoors, before the weather begins to cool back down.

During the first few days, avoid rainy, and especially windy days. Your young transplants do not need to endure a strong spring lightning storm before they are out in the garden.

Observe the changes to your transplants. During the early days of the hardening off process, you may see little change in plant growth. On close examination, you will likely see the main stalk of the plant is growing stockier, which makes it stronger, and helps it to endure wind and weather.

Each day, allow the plants to remain outdoors for increasing periods and allow them to experience greater temperature and wind levels.

Always bring seedling transplants indoors, if frost or extreme weather is forecast.

As planting time nears, after the last frost date for your area, allow your transplants to remain outdoors and uncovered, as long as the nights do not get too cold.  

A cold frame is a valuable tool for the hardening-off process. It allows you to keep your transplants outdoors, except during an extreme period of freezing weather. The cover is lifted each day as the weather warms up. The cover is placed back on, before nightfall.

More Resources Related to Hardening Off Seedlings

Transplanting Seedlings – how to

Transplanting Shock – causes and how to avoid transplant shock

Damping Off Disease causes, signs, how to avoid it

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