Hardening Off the Seedlings

A key question for any gardener—particularly on a nice sunny day—is when to plant things outside. Here in Western NY, our average high temperatures have been in the upper 60’s to low 70’s, with nighttime lows between 48 and 55. As I am typing this, it is 2:30 am (I have always been a night owl) and it is 49 degrees here.

I’ve had herbs, marigolds, zinnia, nasturtiums, forget me nots, broccoli, Romanesco, chard, lettuce, beets, potatoes, and strawberries outside for a few weeks now. Most things are doing really well, so naturally I’ve been toying with the idea of getting everything else out there as well.

This Green Zebra is likely saying "Who do you think you're calling a seedling?"

 It’s important to prepare your plants a bit before thrusting them out into the harsh winds and intense sunshine. This process of gradually introducing the plants to outdoor conditions is called “hardening off.” There are any number of ways to do it, but all take into account three main concerns:

1) Temperature: Most people start seedlings in a very warm place (often 70 degrees or more all the time), so suddenly putting the plants into 50 degree temperatures could shock them. This is why you need to gradually expose them to cooler temperatures, which can be done by leaving windows open (if feasible) or putting them outside for a little while each day to get used to cooler temperatures.

2) Light: It sounds strange at first, but although plants need sunlight, those begun indoors under grow lights are not accustomed to the harsh brightness of direct sunshine. Even the dual T5HO aquarium grow lamps (which are several times brighter than a dual florescent strip lamp) that I use are nowhere near the brightness of a cloudy day, let alone a direct blast of sunshine.

You have to expose plants’ tender young leaves to direct sun gradually, so as not to scorch them, which “bleaches” them and turns your healthy young green plant into something sickly and ET looking. Luckily, shade and a little TLC can usually fix this, but why put your plants through that?

3) Wind and rain: Wind is a big concern, especially for me since it seems like the weather here has been almost constantly windy lately. Remember that your seedlings that have enjoyed the relative calm and warmth of the indoors have not grown up with 15+ MPH winds and pouring rain thrashing them around all day long. If you can, use a fan to provide breeze indoors and begin to toughen up the stems, or leave windows open on windy days to give them some wind exposure. You would be surprised how quickly a fan and/or breeze from the window will toughen up the plants’ stems.

Remember how you tenderly watered your plants with a gentle stream from  watering can or handheld pitcher? Well, Mother Nature will water them by sometimes battering them with heavy rain. This is another reason to toughen them up with a fan before they go outside.

 

I have been hardening off the plants in the seedling room by taking a number of steps. The room is a sunroom without any heat vents, so in the winter when these plants were germinated it was rarely above 62 or 63 degrees during the day, and often around 55 or so at night. As a result, they have grown up relatively cold hardy. This is a huge advantage now, as they are less susceptible to shock from cool nights once they go outside. Lately, I have also been leaving the windows open in there, even on nights where the temperatures got down to the upper 40’s. As far as wind goes, it has been quite windy here lately, so the wind coming in the windows has done a decent job of toughening up the plants a bit. I have supplemented that by using a box fan on a couple of occasions. Last night it got down to 54 degrees in the seedling room, so leaving the windows open has been a great way to begin toughening the plants up to the cool nights. Of course, this is not as feasible if your seedlings are in your living room or dining room.

This room has been amazing for raising seedlings (there is also a window to the left across from the one on the right)

 I like to plant on a cloudy day. That way, the plants get a day to adjust to their new containers outside without being assailed by the intense sunlight right away. The next day I keep them in part shade. The biggest issue with putting crops outside in zone 6A now is the remote possibility of frost, so if you decide to plant in the very near future, be aware that you might have to protect plants if temperatures drop below 40. In terms of potential harm to the plants, nighttime temperatures in the 40’s (especially the high 40’s) won’t hurt your tomatoes, eggplants, peppers, or okra, as long as the daytime temperatures are in the 60’s or above. At worst they just won’t grow much. They will be readying themselves for a nice growth spurt the minute conditions become optimal, though!

Here is my current schedule for hardening off and planting the peppers, eggplants, tomatoes, and okra. Really this is a little conservative given the work I’ve already done to acclimate them to the wind/cool nights, but I would always rather be on the safe side.

Thursday 5/10: 3 hours outside in part sun, protected from direct wind

Friday 5/11: 5 hours outside in part sun, protected from direct wind

Saturday 5/12: put outside in early afternoon (plants spend 1^st night outside)

Sunday 5/13: move to mostly sunny spots on porch (plants remain outside overnight again)

Monday 5/14: Plant! If it seems like an individual plant needs more TLC, then I will move the entire container to a shady spot. That’s the biggest advantage of container gardening! Really, you could get away without doing any pre-hardening if you are prepared to move the containers around as needed. I prefer not to have to do that, but if you’re in a hurry to plant just make sure your plants are cold hardy and go for it! Monday’s forecast is 67 degrees and mostly cloudy, which I find perfect for planting.

Of course, there are as many theories about hardening off as there are people who do it. If your plants have only experienced 70-75 degrees inside and have never felt a breeze then you will want to do a little more to toughen them up before sending them off into the outdoor world. If you’re unsure, keep some plants in reserve in case some of the first plantings don’t make it.

And remember: part of the fun of gardening is experimenting with different techniques and seeing what works for you. Tomatoes, peppers, eggplants, etc. are pretty tough, and can survive some pretty harsh conditions. I know people who planted container tomatoes outside in mid-April here. It just takes a lot more protection and maintenance than the normal gardener would be prepared or willing to provide. If you rush things too much, your plants will usually survive, but they will often just sit there and look sad until the weather improves to optimal conditions for them.

If you take a few simple precautions, and provide a little extra care at the beginning, you’ll have an amazing garden before you know it!