How to Harden Off Seedlings
March into April always signifies a major ramp up in sowing seeds. This year is no different but it feels like I have so many more seedlings in the basement than I did last year at this time. I’m full and may or may not need more shelving units. Yet already tight on space… Oh me, oh my! My next big project after ironing out my big-girl irrigation system is a propagation greenhouse! It must happen by next season. I’m quite literally outgrowing my basement.
When sowing seeds indoors, eventually you’ll need to harden them off. It’s my least favorite seed task because it’s a major time suck for me especially with working off the farm. Every day, in and out of the basement carrying trays in each hand. Back and forth. Back and forth. And the number of trays just keeps increasing each week. This week, it’s the 30+ trays of cut flower gardens that are being carted in and out by hand plus my trays of late cool season annuals and the start of the warm season. For the past five years and to this day, I grow every seed in my basement. This is all a very protected environment, unlike the conditions outside in which my seedlings will need to survive.
So what is ‘hardening off?’ Hardening off entails slowly getting seedlings adjusted to the outside environment; this includes not just temperature but also sun and wind. All three are important to take into consideration. By getting them used to these conditions in a slow and methodical way, you increase your chances of survival and having healthy plants that grow into the flowers or vegetables you’re hoping for.
My method lasts about a week and starts with adjusting the seedlings to temperature first. I place seedlings in an area outside that is shaded from the sun and mostly protected from the wind. I do not have a screened in or covered porch but this would be an ideal place to start your hardening off process. Quite honestly, I rely heavily on the shaded side of my house.
On day one, I start with a couple hours of outside temperatures and increase by a couple hours each day. On about day 3, I start adding a couple hours of sun with increasing time in the sun and cold each day. Finally, I add wind by placing them in an area that gets some cross wind. I let them outside for a night or two (usually more because I get behind with planting!) to round out my process. By about a week of slowly increasing all three variables, the seeds are ready to plant.
Many people are surprised to hear about the affects of the sun and wind on little seedlings. In my experience, temperature is typically the easiest variable for seedlings to adjust to. After planting out, for an added layer of protection and some more time to adjust, you can add a light frost blanket (also known as remay or agribon) for a week or two and then remove. This will also help prevent critters (deer and rabbits mostly) from nibbling tasty and tender baby plants. To do this, you can purchase wire hoops to hold the fabric off of your plants then use bricks, rocks, or weights to place around the edges to keep the fabric from blowing around. It lets in sunlight, air, and water but at a reduced intensity helping prevent sun/wind burn. It also adds a few degrees of protection from cool night temps in the spring (but can be used all through the summer in this way).
Below is an example of sunburn or wind damage to a small seedling. Often they can recover but sometimes it will kill them before they even start to grow.
Other things to keep in mind when acclimating seedlings to the outside:
Make sure seedlings are well watered before setting them out each day. They dry out much faster outside and this causes unnecessary stress.
Make sure your seedlings are big enough for transplanting: the seed packet should tell you how many weeks a seedling needs to grow before being transplanted. Too small and they usually stand no chance. Too big and they can have a rough transition, especially if they have become root bound.
While it’s super easy to kill baby plants (I do, all the time!), they are also generally more tough than we think and can recover from quite a lot of stress.
Bottom line, don’t let anything hold you back! If you’ve been wanting to try seed starting, just try it! The worst that can happen is you kill a little plant.
Happy gardening!