Q. I inherited a greenhouse, which will remain unheated, in the garden of a house I have moved into. I began sowing seeds indoors at the beginning of the month, and now have a number of growing seedlings. My question: When can I put the flats of seedlings out into the greenhouse?
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Distroscale
A. The timing for each flat of seedlings will depend on the type of seedling and the weather.
What you can do now is to get the greenhouse ready to receive the developing transplants. I usually sweep out and wash down the interior, and give the shelving a thorough scrubbing. Then I close the door and hope for a sunny day or two to administer a bit of a heat treatment to the interior.
I keep a minimum-maximum thermometer in the greenhouse. That gives me readings each morning of how low temperatures are falling at night within the structure. I usually begin the shift of flats to the greenhouse around now, but weather patterns have become so wonky in recent years that I’ll be guided by the thermometer.
I begin moving the hardiest types of seedlings into my unheated greenhouse once overnight temperatures no long dip below at least a few degrees above zero. In the first wave of plants to make the move are leeks and onions, pansies and violas, cabbage family vegetables, sweet peas, snapdragons, endive and escarole, and the hardier types of lettuce.
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The first lettuce to be placed in the greenhouse is usually Tom Thumb — a longtime favourite for its easy-growing, hardy nature and for the small butterhead lettuces fully packed with succulent cream and green leaves. Tom Thumb is an heirloom dating back to 1830. My seed source is West Coast Seeds. The Little Gem type miniature romaines have also done very well for me in cold weather.
By April, it is usually safe to place flats of warmth-loving plants in the greenhouse.