Hardening Off Seedlings (Seed Sowing 101)

Once your seeds have grown big enough to plant out in the garden it’s time to find a way to get them into the garden.  Direct sowed seeds have a big advantage in this area as they have grown from the start in the great outdoors are are already well adapted to the weather.  Seedlings grown indoors aren’t so lucky.  Bringing those seedlings outdoors too fast may result in planting shock because of the change in intensity of light or the weather conditions.  The method you use to transfer your seedlings outdoors is called hardening off.  Hardening off is where the plants are gradually adapted to the outdoors over a period of several days.  Each day the plant is exposed to more of the natural weather until it is ready to grow outside by itself without protection.

How to Harden off Transplants

The most common way to harden off seedlings for transplanting is to place them outside in a protected location for a short period of time then bring them back indoors. Each day increase the length of time until you feel they are ready to be planted in the garden. Start with 2 hours on a mild morning and add an hour or two each day. Keep the seedlings well watered at first them back off as they get stronger.  If your transplants start to wilt bring them back inside to recover.  Another method is to move the plants into a shady location for a while then expose them to a little sun each day, again increasing in time, until they are ready.  Of course shade loving plants won’t need exposed to the sun!

Another method is to place the seedlings outside under some sort of cover.  A folded piece of cardboard is a good option.  Each day remove the cover for a period of time that increases each day until the seedlings are hardened off and ready for transplanting.

When is the Ideal Day to Plant Seedling Outdoors?

I like to watch the weather for the ideal planting day to put my seedlings outdoors.  A series of overcast days in a row with some scattered showers are perfect!   Overcast days are great since the sun exposure is limited for planting a new plant in the ground.  I also try to wait until evening to put new plants out since it will give them time overnight to adapt to their new homes.   The safe planting date in our area (Spring Hill, TN) is usually mid April but it is often better to wait until the end of April before putting summer vegetables and annuals outdoors.  That two week period between April 15th and the 30th is a very tricky period of time weather-wise and the plants make up that time very quickly if planted later.  I’ve had tomato plants planted right after the frost date not do nearly as well as those planted later.

When you put together your seed starting plan for the year don’t forget to add in a few days of time to harden off your seedlings.  You (and your seedlings) will be glad you did!