What Does It Mean To Harden Off Plants?

So, you're pretty proud of yourself. And why shouldn't you be? You have successfully managed to start seeds indoors and grow them into healthy plants that are ready for the great outdoors — or are they? Well, not quite yet. There is one more vital step you need to take before permanently moving your tender seedlings outside. You need to harden them off.

Think of hardening off plants as training them for whatever nature may throw at them. Wind, rain, and bright sun can be detrimental to seedlings that have not been properly acclimated. Although the hardening off process does not protect plants from frost, it prepares them for the outdoor weather conditions they will face daily. If you move seedlings straight from their protected homes indoors into the garden, the sudden change in conditions could ruin all the tender loving care you have shown your plants until now.

Hardening off seedlings to acclimate them to the outdoors takes a week or two, so keep that in mind before you decide when to plant your garden. Once your seedlings are large and healthy enough to move outside, it's time to start the transition a little bit at a time. The appropriate size for transplanting varies depending on what you are growing, but it is generally 6 to 8 weeks after starting the seeds.

The best way to harden off your plants is to leave them outside in a sunny or slightly shady area starting with one hour on the first day, then bring them back indoors to their regular spot. On the second day, leave them out for two hours. After that, increase their time outside by one hour a day until they have been outside all day. At this point, you can leave them out overnight. Once they have spent a night or two outside, they are ready to plant.

If you are not home to move plants in and out during the day, you can place them in a shady, protected spot while you are gone. A place under a tree or on a covered patio protected from the wind is ideal. Then, each day, move them slightly further away from the protected area, so they get a little more sun and a bit of wind. Bring them in at night until they are in full sun without protection.

Never leave your seedlings out when heavy rain, hail, harsh wind, or temperatures below 45 degrees are predicted. Hardening off can be a slow process, but it is a step that cannot be skipped. Moving from grow lights to the bright sun is a huge adjustment, and going straight from one to the other will cause sun scald. Likewise, going from a calm indoor environment to strong winds can cause broken stems or leaf damage. Since you have taken the time to grow plants from seeds, it is worthwhile to make sure they get the best start as healthy plants when you place them in your garden.