Time to Harden-Off Tomato Seedlings

It is approaching time for everyone who has tomatoes growing in their windows to start “hardening-off” the plants. Basically this is a process of gradually toughening up the plants and introducing them to outside. The tomatoes have been growing in the protected environment of your windows and the outside is going to introduce a whole new set of growing conditions including direct sunlight, wind, lower temperatures, and air pollution. If you were to take the plants directly from your window and just plant them outside, their growth would be significantly slowed or they may even die.

The first part of “hardening-off” is to toughen your plants up by reducing the amount of water they are getting (but don’t cut off water completely), and if possible by reducing the temperature by a few degrees, for their last week in the window. The result will be a shorter, more fibrous plant that will suffer less when it transitions to outdoors.

The second part of “hardening-off” is to acclimatize the plants to outdoors. This is a gradual process where you take them outside for longer and longer periods. This can take up to a couple of weeks and it is best to be patient since introducing them to outside too quickly may do more harm than good to the plants. Here are a few things you will want to consider during this process:

It is best to start with only a few hours of sunlight, or if possible partial shade, and then increase the amount of sunlight gradually so that in 7 to 10 days the plants can do a full day of sunlight.

Remember that the outside will speed up the rate that the soil will dry, so be sure to keep the soil moist enough to prevent wilting.

Wind can damage the plants by breaking the stems of un-staked seedlings, or by whipping the seedling around to the extent that the roots are loosened and then damaged. Therefore it is important to choose as sheltered place as possible for the first week of taking the plants outside. If you don’t have a naturally sheltered place to put them you may consider using a “cold-frame” system to keep them sheltered: I am taking my tomatoes out in larger Rubbermaid bins without their lids for the first week. This way they get the sun and outdoor temperatures with less risk from the wind.

Cold temperatures can pose a serious threat to tomato seedlings. Be sure to bring the plants indoors if the temperature is going to drop (especially if it is below 10C) since this will put the plants into shock and seriously slow down their growth if they are fortunate enough to survive.

The air quality in southern Ontario is far from perfect and unfortunately smog can do a lot of damage to sensitive plants. Therefore, it is important to consider the air quality when transitioning your plants to the outside. Ontario has a great smog warning system and you can sign up to get smog warnings for where you live to be sent to you by e-mail (go to http://www.airqualityontario.com/alerts/signup.cfm to sign up). If it is a smog day, like today, it may be best to keep the plants inside or reduce the time they are outside.

You may also need to stake the seedlings, especially the ones that are tall and thin. Last year I used twine and old chopsticks (but you could use almost anything a similar size). I found it worked best if I put the chopstick into the pot as far away from the plant as possible (to prevent damaging the roots), tied one end of the twine to the chopstick, loosely looped the twine around the plant to provide some support, and then tied the other end to the chopstick. It is important not to tie tightly around the plant because it may cause damage; instead you want it so that the plant has some room to move and grow while also having some support.

If you do not have a method to take your plants outside, you can either contact me directly or send a message over the list to see if there is someone with a backyard or balcony interested in caring for the plants for the last few weeks before they are planted in the garden.

If you have any further questions or comments, please feel free to contact me