You’ve started your seeds indoors, witnessed the magic that is germination, and watched your sprouts grow into lean, green little seedlings. You think they’ll be ready to enter the world outside the confines of that warm, cozy bubble called your kitchen (or bathroom or greenhouse or wherever they’ve been shacking up) in a few weeks. But like a good parent, you don’t want to set them out before preparing them for what it’ll really be like out there.
You need to harden them off first.
It sounds kind of gangsta, to harden off your seedlings, but that’s exactly the attitude they’ll need when they make the transition from being indoor to outdoor plants. Hardening off your seedlings is a process of acclimating them to the elements to assure their survival.
Up until this point, your seedlings have been protected from wind, rain, cold, heat, and the intensity of sunlight. They need to toughen up before you throw them into the deep end of the pool, so to speak.
I actually start hardening them off when they reach a couple of inches tall. Every day, I lightly brush my hand back and forth across the seedlings to simulate a breeze. This action strengthens their stems and prepares them for the harsher winds they’ll face out in the garden.
Once nighttime temperatures are consistently in the mid-50°Fs, I start to acclimate them outdoors by placing them in filtered or partial sunlight for a few hours in the afternoon. This could be under an eave, a covered porch, or a shady tree. Before the sun goes down, I bring them back inside. The next afternoon, I put them outside again and bring them in at night. This happens for two to three days as the seedlings adjust to their environment.
Over the next few days, I gradually leave them outside for an hour or two longer each day, in brighter and more direct sunlight, until they can finally spend the whole day and the whole night outside.
The entire process takes 7 to 10 days (and sometimes up to 14 days, if nighttime temps take a sudden plunge). Always be mindful of what the weather’s doing during this time. If the wind picks up, shelter your seedlings so they don’t snap in half. If the sun is especially hot, keep the potting soil consistently moist so your seedlings don’t dry out or suffer undue stress.
So what happens once your seedlings are hardened off? They can simply live outside (and be repotted as needed) until you’re ready to transplant them in the garden. Or, you can transplant them right away on a mild, cloud-covered day to ease them into their new home.