How to Harden Off Seedlings For Planting

Hardening off seedlings for planting is crucial for the transition to the garden.

What is “Hardening Off”?

A new gardening season is here and everyone is thinking about starting seeds. The process of preparing young seedlings for transplanting outdoors is the turning point for gardeners who are starting seeds for the first time. When you don’t know you have to harden off seedlings, you are likely to end up with a high mortality rate. If you try to harden off seedlings that are weak to begin with, you may end up with similar results. So I’ll back up a little…

Before You Start Hardening Off

Once seeds germinate, they need light, moisture, air, and warmth. To prepare them for their permanent home, the goal over the next few weeks is to establish a strong root system and stocky, leafy top-growth. Gradually move them from the perfect conditions in which they sprouted to the relatively unprotected environment of the garden while providing healthy growing conditions all along. This process is known as hardening off. 

New seedings in a black tray.

Light

As soon as the seed leaves emerge, young plants need as much light as you can give. If you are using artificial light of any kind, keep seedlings within inches of the bulbs for twelve hours each day. Flooding the plants with light helps them grow short and stocky rather than thin and rangy.

Moisture

When watering overhead, use a fine mist or fogger. If the soil dries too much with this type of application, you can water by placing the seed tray, peat pots, or soil blocks into a shallow tray of water, allowing the water to wick upward in the soil. Misting works well for the first week or so, later on bottom watering ensures better deep watering, which is key to good root development. If the soil mix does not contain plant food, provide it in the water.

Air

Good air circulation is important from the start. Remove humidity covers used over trays during germination within a day or so. Keep a small personal sized fan on the young seedlings (on its lowest setting) to help keep plant growth in check. This will result in stockier seedlings that adapt more easily to natural outdoor conditions. The fan can run on the same timer as the lights.

Warmth

Ideal germination temperatures are often not ideal for producing strong transplants. Tender plants like tomatoes and peppers should be kept in a place that stays above sixty degrees. Hardy vegetables like kale and lettuce can grow in places with temperatures in the fifties. There is evidence that suggests warm soil and cool air together may be a good combination, but most of us don’t have that degree of control. As the time approaches for hardening off, providing five to ten-degree fluctuations in daytime and night temperatures is helpful.

A calendula seedling growing in a soil block.

How to Harden Off Seedlings

It’s called hardening off, but it should happen gently. Always water the seedlings well prior to leaving them outdoors, and check on them periodically while they are outside. Pick a calm day when the temperatures are close to those in the seedling’s current environment. Take them outdoors to a shaded place for an hour or so.

The next day, if conditions are similarly mild, take them out again, this time for several hours. The day after that, take them to a spot with direct morning sunlight for an hour and shade the rest of the day. Each day add a bit more sunshine to their outdoor time. This process may take a few days or a couple of weeks, depending on how different the indoor conditions are compared with those outdoors. The last night prior to planting, they should be left outdoors.

Following these tips will ensure your babies are tough enough to handle real-life conditions when the time comes. That means you’ll have a bit less stress too.

Make easy plant labels for your seedlings!

Hardening Off Seedlings FAQs

How big should seedlings be before hardening off?

I like to grow seedlings indoors until they have at least four true leaves, which takes about a month. The hardening off process starts after the first pair of true leaves have grown in. The second pair will grow during the transition.

Will leggy seedlings still grow?

The short answer is yes, but leggy seedlings may flop over and stick to the soil after rain or irrigation after moving to the garden. They will become stronger within the first week or two after planting. It’s best to avoid this problem.

Seedling stems stretch to find sufficient sunlight and become leggy either due to a lack of light or as a means of competition as the true leaves develop. Increase sunlight and air circulation to build strength before planting them outdoors. Thin to a single plant per cell as soon as the first true leaves begin to appear. Space the seedlings so that the leaves are not touching or blocking one another.

At what time of the day should seedlings be transplanted and why?

Transplant seedlings in the late afternoon or evening, or on a cloudy day. Although a few overnight hours may not seem like much time, young plants begin adapting immediately. The first full day in the garden will be less stressful if they have a few hours to ease into the new surroundings.

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