I grew summer-flowering annuals (dahlias, zinnias, salvia, and marigolds) from seed and would like advice for when to start hardening them off outside. I know my tomatoes and peppers need more time inside, but can I start to acclimate the flowers soon?
— Anna Chung, Wilmette
It sounds like you’ve chosen a nice assortment of annual flowers and fruits to start with seed at home. Growing plants from seed is a great way to get access to plant varieties you can’t find in a garden center. Tomatoes and peppers are botanically classified as fruits but many gardeners refer to them as vegetables and use them as such in the kitchen. It’s hard to beat the taste of vegetables grown in your own garden.
Start by establishing a planting date for the plants. To determine that, use the average last frost date for your area as a guideline. I was recently asked what date the Chicago Botanic Garden now considers the average last frost date — it has been May 15 for as long as I can remember with one surprise year where we had a hard killing frost on May 22. After doing some research and getting frustrated by how wide a range of dates there are, I have decided to stay with May 15 as the average last frost for the Garden. Each spring the average can vary and different areas in Chicago can vary too. It’s best to monitor the long-range weather forecasts before making a final decision on when and what to plant. With all that in mind, it’s time for you to start hardening off your flowers for installing in your garden. I generally wait until after Memorial Day to plant tomatoes and peppers since they’re sensitive to cool night temperatures. I like to wait to start planting summer annuals in the Glencoe area until the last week of May.
Hardening off plants is the gradual process of acclimating them to outdoor conditions after growing them under lights in your home or in a greenhouse. The plants need to slowly adjust to increased sunlight, wind, fluctuating temperatures, and varying humidity to avoid damage. Plan for a seven-to-ten-day process to fully acclimate them before installing them in your beds or containers. Start by placing the plants in a shady area with no full sun for three or four days. Then gradually increase their time in the sun over the next week. Monitor moisture levels as they may begin to dry out more quickly as the sun exposure increases. Also keep an eye on outdoor temperature. Some of your plants may struggle when temperatures dip into the 40s, especially as they’re still adjusting to being outside. A cold snap can set them back even after hardening off. If overnight temperatures drop below 50 degrees, move them to a garage or bring them indoors.
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For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.