Hardening Seedlings off

If you haven’t already started, it’s time to start hardening off those seedlings. The days are getting warm and evenings are pretty mild. Hardening off (if done properly) takes several days to complete. Start slowly. While it’s tempting to rush outside and soak up every inch of sun, that is the quickest way to end up with a nice red burn (or sunstroke). Just like you need to get used to the warm weather and sunshine, so do your seedlings. These seedlings have no experience with direct sun, wind, varying temperatures and drying soil.

Seedlings in their trays, ready to be hardened off

On a mild day, take your seedlings outside in a sheltered spot and leave them for 2-3 hours. If you don’t have a good shelter you can cover your seedlings with a shade cloth or a milk crate. Something with holes to allow filtered light, wind to pass through is best. After that time bring them back indoors. As you increase the time they are outside, start moving them into direct sunlight. This will slowly get them used to direct sunlight, wind and variable temperatures. Continue this process for about a week. Water availability is another stress your seedlings have no experience with. Up to this point your seedlings have been watered regularly, not too much and never too little. When hardening the seedings off, go ahead and start reducing the frequency of your waterings. Never let your seedlings severely wilt, but allow them to dry periodically. This will cause their roots to grow deeper looking for more water.

So why do this? Hardening off your seedlings forces them to grow a thicker cuticle (a waxy outer covering which will reduce water loss), grow more vigorous root systems, and thicker stems (which reduces lodging due to wind). The idea is to give them just a little bit of stress to force change, without causing them permanent damage. The plants should also turn a nice dark green during this time as they produce more chlorophyll to protect against the sun.

Nice dark green leaves of the zucchini after living outside for a week.

Be aware, even properly hardened seedlings can still be damaged by cool temperatures. Warm weather crops like peppers and eggplants can be damaged by night temperatures below 60 °F. Cool weather crops can handle colder temperatures, and eventually can handle light frosts. If a frost is expected, and you don’t want to bring them indoors, cover them with a sheet. The sheet provides a protection that helps keep a layer of warm air around the seedlings, preventing frost from forming. 

Another thing to watch for is bleaching. This is when the leaves of the plant turn white in response to sun damage. You turn red; your plants turn white as the chlorophyll dies. In a nutshell, there is more intense light than the plant can photosynthesize. So in response, the plant reduces its ability to undergo photosynthesis. This results in damage to the chloroplasts (those little organelles that make plants green). Without chloroplasts, there is no chlorophyll and the plant turns white.

As always, this post was based on both personal experience and these helpful references:

http://gardening.wsu.edu/library/vege003/vege003.htm

http://www.coopext.colostate.edu/TRA/PLANTS/hardenof.shtml

http://www.uidaho.edu/~/media/Files/Extension/Bonner/ExtensionSpotlight/30HardeningOff%20Plants.ashx

Adamska, I ELIPs - Light-induced stress proteins: Physiologia Plantarum 1997. 100:795-805