The Importance of Hardening Off Your Seedlings

“Hardening off” a tray of seedlings refers to the process of gradually exposing them to the outdoor elements to ensure a smooth transition from greenhouse (or indoors) to the garden. While it can be a bit time-consuming, it’s an absolutely necessary step to ensure that the seedlings stay healthy and can tolerate wind, rain, and significant temperature fluctuations.

Why Do We Have To Harden Off Seedlings?

It’s extraordinarily tempting to skip the process of hardening off seedlings, especially because it occurs during one of the busiest times of the year for gardeners. However, without it, there is a tremendous risk that the plant babies you have carefully nurtured indoors will not be able to survive the transition to the garden. It’s an integral step to ensure that your tray(s) of seedlings have the stem strength and overall development to tolerate the outdoor elements.

When to Harden Off Seedlings

It’s best to start the process of hardening off your seedlings at least a week before they get transplanted outdoors to the field. This will allow enough time to gradually expose them to the outdoor weather. For tender annuals (meaning that they cannot tolerate a frost) here in Greater Philadelphia, this process will typically begin between late April and early May. For hardy annuals that can handle colder temperatures, this will likely occur sometime in March.

Just as importantly, be sure to check the forecast! During the first few days of the hardening off process, it’s especially important to ensure that there is no extreme weather forecasted as you acclimate your seedlings to the outdoors. High wind gusts, significant rain, or unusually low temperatures can all be damaging to your seedlings. While they will invariably experience these elements after their transition to the garden, they have not yet developed the stem strength to tolerate this, so treat them with care as you begin the hardening off process!

How to Harden Off Seedlings

The process of hardening off seedlings is essentially as simple as bringing the tray(s) outdoors for a gradually increasing amount of time each day. The goal is to slowly (!) expose them to the outdoor elements, so that first day you may wish to set them on a covered patio or a more protected area for an hour or two. The following day, let them sit outside for a little more time, maybe a bit more in the sun this time. By the fifth or sixth day, the trays can stay outside overnight assuming the temperatures won’t dip too low. After about a week, the seedlings should be ready to stay outside for a full 24-hours and begin getting transplanted to the garde

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A couple trays of sweet peas, getting ready to go from the greenhouse to the garden