Seasonal News

Seasonal News

Spring Rituals: A How-To Guide for Hardening Off Seedlings

Author, Sara Post, active Master Gardener

If you like starting your plants yourself by seed then you undoubtedly have many already happily sprouting away in their seedling trays and some may even be beginning to look like they are ready for some new space! It is time then to harden these babies off before sending out into the great world to fend for themselves. This step is called “hardening off” because it is doing just that: slowly exposing your seedlings to slightly harder and less cozy conditions, compared to the safe haven given to them during their initial sprouting seedling stage. We do this step until they are well adjusted enough to better tolerate swings in temperature, hard wind gusts, harsh hot sun, and changes in moisture levels. 

I have come to learn that with some planning and observation, the hardening off process can be quite easy. First and foremost, I no longer try to harden off my tender seedling crops too early. Meaning if in my growing zone 7 has predicted the last frost date is set to be April 15th, then I will time my seedlings to be ready for hardening off no earlier than that date. Trying to get a head start by testing your luck with the weather will not really get you that far ahead. Growth will potentially be delayed if the seedlings experience too cold of temperatures and even worse totally wiped out during that one tiny sudden dip in temperature when you weren’t looking and you have to start all over. And studies show that even if you get extremely lucky pushing the envelope to start early, you don’t really gain much. Seeds started at their more ideal time are usually more robust and catch up quickly, sometimes even outgrowing those started first being that much healthier and with more ideal conditions.

  1.  Correct timing: plan according to your growing zone and last frost date counting the number of days your seedling will spend in the seedling tray before the hardening off process begins.  There are three last frost dates in Virginia according to the maps.  Here in the Roanoke Valley it is May 15th.  Ask your local Extension Agent for the proper date if you are not in this area

2.  Know the ideal growing conditions of the plants you are growing. Shade loving or partial shade loving plants will do best as seedlings and avoid hardening off in full sun. Sun loving plants need to be very gradually exposed to full sun as seedlings so they have time to adapt.  

3.  Choose the appropriate hardening off environment: ideally one with protection from the wind, includes areas of partial shade, receives gentle morning sunlight as opposed to the hotter afternoon sun, and is protected from critters trying to munch or step on your babies.

4.  Start by moving your seedling trays (or potted up plants) on day one to the protected spot with partial shade and plan for them to be there for only a couple hours.

5.  Increase the time by one to two more hours on day two and then bring them back in again. Continue increasing the length of time by a couple hours each day in the protected environment. By about day 4 or 5 you can choose to let them spend the night outside, especially if very mild nighttime temperatures are predicted.

6.  After about 6-7 days of gradually increasing the time spent in the protected environment, begin allowing more direct full sun on the seedlings (only for the sun loving plants, the partial shade and shade loving plants will be happier staying out of direct sun). Continue for one or more days (sometimes up to two weeks) giving your sun loving seedling plants more direct sun for most of the day.

7.  Throughout this process be sure to check often for signs of sun burn on the leaves and moisture levels. Adjust conditions promptly to avoid too much stress on the seedlings. Once they have gone through these steps, they should be ready for their permanent homes in the garden!

Options for a protected environment could include a mini greenhouse, mini hoop house, pop-up plant tent, cold frame, straw bales with old windows on top, and even just an outdoor table under an umbrella. I have used an area of my backyard that receives the gentle morning sunshine under an awning with some shelving providing a windbreak. I gradually moved the seedling trays into stronger areas of sunshine using a temporary table on sawhorses. Be sure to provide plenty of ventilation and check moisture levels during hot afternoon temperatures. Do not underestimate the strength of the sun on tender new seedlings. It is much stronger than any indoor grow light by far. Following these steps and guidelines, you will be well on your way to end up with very healthy plants becoming their most beautiful and lush selves.