A Practical Guide to Hardening Off Seedlings

Once your seedlings have filled out their pots, it’s almost time to plant them into your garden. But do not make the mistake of simply taking them from their indoor growing area and immediately planting them outside without shelter. Doing so risks killing or severely stunting the growth of your plants. Rather, you must acclimate seedlings to outside conditions, a process called hardening off.

I’ve seen a lot of unnecessarily-complicated hardening off protocols, so I want to present my approach, which is both easy and scalable. But first, let me explain why we need to harden off plants.

Why Do Seedlings Need to Be Hardened Off?

Generally speaking, there are three environmental stressors to be wary of when transitioning plants from indoor to outdoor conditions:

Sun damage. Sunlight is very intense; you wouldn’t leave a newborn baby out in the sun all day without protection, and you shouldn’t expose your plant babies to excessive sun either. Plants grown inside have lived very sheltered lives, where they get just the right dose of light, devoid of harmful UV rays. Plants deploy a variety of UV-protective mechanisms, but they need a little bit of time to ramp up those mechanisms, so don’t give then too much sun too soon.
Drying damage from wind. For seedlings, wind can be highly destructive. Just as a high-power fan can quickly dry out a wet floor, wind can quickly suck all the moisture out of your plants’ leaves, efficiently killing them. I’ve found that fleshier leaves are more sensitive to this. So a succulent cabbage seedling or snapdragon is very susceptible to wind damage, but grasses are generally fairly immune to wind damage.
Damage from extreme temperatures. Your plants are acclimated to your home’s consistent temperatures. Very cold or very hot conditions can damage your plants. I usually avoid exposing seedlings to freezing temperatures, but I tend to find that temperature, on its own, is not as destructive as sun and wind.

My Quick and Dirty Hardening Off Protocol.

When hardening off plants, you want to gradually increase their exposure to sun, wind, and extreme temperatures. This allows the plant time to adapt physiologically so that it can handle these environmental stressors. The internet is filled with laborious hardening off protocols; I’ve seen it suggested that I ought to move my plants outside and inside on a daily basis, giving them increasingly long intervals of exposure to the outdoors. I have thousands of seedlings, so I really can’t be bothered to do that.

To efficiently harden off my plants, I use row cover. Row cover comes in many weights, and for hardening off I use a lightweight option: 0.55oz per square yard. This is equivalent to Agribon-19 or DuraSpun 55. When you cover your plants with row cover, you raise the humidity around the plants, decrease wind, increase the temperatures, and reduce sunlight (Agribon-19 and DuraSpun 55 block 15% of light, and I suspect they block out a substantial amount of UV). So row cover creates the perfect microclimate for your plants. I typically do my hardening off in two steps:

Place plants outside, under row cover, in a part-shade location (3-6 hours of sun per day) for 3-5 days (see Figure 1). If the sun is very intense, use a double layer of row cover (thus blocking 30% of light) for the first 2 days.
Remove row cover, leave plants in place for another 3-5 days. I usually remove the row cover on a cloudy day when there isn’t too much wind.

After 6 to 10 days, I plant my seedlings to their permanent location. I prefer to plant on cloudy days. This is not a particularly precise method, and I have so many plants that I usually forget about some, and they get substantially more (or less) hardening off than this. It really doesn’t matter. The important thing is to check on your plants. If they get wilty, cover them back up with row cover. If they’re covered with row cover already, bring them inside. Get to know your plants, they’ll tell you what they want.

Figure 1: I’ve wrapped my tray of plants in lightweight row cover, tucking the edges under the tray so it doesn’t blow a way. It doesn’t look very elegant, but it works.

After transplanting into the garden, I usually protect many of my plants with row cover for two to four weeks. This keeps squirrels out, and it also coddles them a bit as the roots develop. If you choose to use row cover after transplanting, be mindful of two things:

Make sure your plants aren’t overheating. If the temperatures are in the mid 70’s, and the sun is shining, it might be very hot under the row cover. Some plants will like this and some will die. So stay vigilant.
Since row cover shelters the plants from wind and sun, your plants may be slightly shocked when you remove the row cover. To reduce shock, I try to remove the row cover on a cloudy day.