Hardening Off Seedlings
Late spring is a transitional gardening period and an exercise in patience. Here in the northeastern U.S., outdoor gardening is limited to cold-tolerant plants like peas and lettuce. In April, I planted peas, carrots, onions, and turnips in beds and containers, and planted lettuce in a cold frame (after I removed it from housing my herbs throughout winter).
The other gardening activity I’ve been doing is indoors. Back in February, I planted seeds in seed trays under grow lights to create seedlings to plant in May. I scaled back a bit this year, but still have about 125 seedlings I’ve grown under grow lights, specifically, fennel, cucumbers, tomatoes, celeriac, and a few herbs.
Before planting seedlings outside in the garden, they need to be hardened off. “Hardening off” is the process of gradually exposing the seedlings to outdoor conditions so that when they are planted, they’ll be prepared to face the elements. While they’re growing indoors, they’re given careful treatment under consistent conditions, which is necessary but it makes them a bit “soft.”
This year has been a bit of a challenge because of our reluctant spring. I’ve been trying to harden off the plants for about 2 weeks now, but the temperatures either don’t get warm enough (it snowed lightly in New Jersey today… it’s May 9th), or it’s rainy and windy. I’m hoping this coming week will be much more conducive to hardening off because these seedlings look ready to plant.
Cucumbers hardening off under the eaves of my garage.
How to harden off seedlings
Start about 1-2 weeks before transplanting to the garden.
Put plants outside on warm days in a protected shady area. Start putting them out for about an hour and then increase the time outdoors by about one hour per day for about 7-10 days.
If the conditions are very windy or cold (under 45-50 degrees Fahrenheit), don’t put them outside. These conditions can kill even the strongest seedlings.
Reduce watering during this time to slow plant growth. (Don’t dehydrate them, just reduce watering a bit.)
These are general rules and if you don’t strictly follow them, and have to improvise, that’s ok. I put the seedlings out yesterday because it got to about 70 degrees outside, but today, we have high winds and cold temperatures, so I didn’t put them out. Tomorrow will be better. The idea is that they need to go through a period of gradual exposure to the elements to be strong once they reach their permanent home in the garden.
