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Setting Out, Hardening Off

Plants raised indoors need to be acclimated before transplanting outdoors. Here's how.

Hardening Off Plants“Gardens are not made by singing ‘Oh, how beautiful,’ and sitting in the shade.” — Rudyard Kipling

My grandparents always called it “setting out plants.” We know the process of introducing our indoor raised or recently purchased greenhouse grown seedlings to the outdoors as “hardening off.” Whatever you call it, the gradual introduction of your tender young plants to the cold, cruel world of the outdoors needs to be done with attention and patience. You wouldn’t just push your children out the door without some experience of what they were about to face, would you? Your plants are like your children. They need to adapt to conditions outside the home.

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Hardening off is the process of acclimating plants to outdoor conditions. In some parts of the country, this process is well under way. In northern settings or places of higher altitude where the possibilities of frosts will continue for another two or three weeks, we’re still waiting. Timing is important. Many garden books will tell you that plants started indoors are ready to go out when its roots have filled the container. But if outdoor conditions are still too cold or wet, your tender plants may be set back. On the other hand, if they’re left in pots and their roots continue to grow, it may set back their growth. Transplanting into a larger pot is called for when outdoor conditions aren’t yet right. Good timing–from when you start your seeds indoors to the time you set them out–is crucial. Here’s a list of minimum temperatures that plants will tolerate when they’re set out in the garden (and some other good tips as well).

When you first bring plants outside try to protect them from direct sun and winds. Leave them out only a few hours a day to start and make sure the soil they’re in doesn’t dry out. Gradually increase the time the plants stay out. After a week (or more) they should be ready. Remember that plants need to adjust to the heat as well as the cold. When setting out summer seedlings (tomatoes, squash) give them time to adjust to the warm days and strong sun they’ll find outside. More tips can be found here.

Some gardeners use cold frame and hoop greenhouses to assist their plants when first set out. Again, attention is important. Make sure the cold frame is properly ventilated so that your delicate green things aren’t exposed to too much heat. Ready to put your plants in the ground? Transplanting is another skill altogether. Any thoughts?

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Eric Vinje founded Planet Natural with his father Wayne in 1991, originally running it as a grasshopper bait mail-order business out of a garage.

Eric is now retired, but is still a renowned gardener known for his expertise in composting, organic gardening and pest control, utilizing pesticide-free options, such as beneficial insects.

Eric believes when you do something good for the environment, the effects will benefit generations to come.

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