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10 Helpful Steps to Overwintering your Plants in a Greenhouse

What is overwintering? Quite simply it is the process of simulating the season of winter for your tender plants in a controlled environment. By carefully controlling the amount of light, water, fertilizer and frost a plant is subject to, you can create a safe ‘winter’ state for your plants and help them survive even the harshest winters to bloom again. As a reward for success, you can use next years gardening budget to invest in new plants and gardening accessories rather than replace your tender perennials.

Overwintering your containers and hanging baskets sounds complicated, but it really is quite simple - follow the easy steps below and you can overwinter plants successfully.

1. Are Your Plants Happy?
Healthy plants are happy plants. Just as the saying is true for spring and summer, it holds even more truth in winter. Your plants stand a much better chance of surviving through the winter if they start out healthy. Some varieties of plants may benefit from one last feeding before going dormant for the winter, so it is a good idea to check with your local nursery or Master Gardener Extension for proper feeding advice.

2. Shoo, fly!
Pests are a nuisance throughout the warm months, and they can continue to be just as pesky in the cold months. Your plants should be pest-free before going dormant to prevent an attack later - when they can't fight back. Inspect leaves carefully before moving any plant into the greenhouse; if a plant looks infested, don't bring it in. Use organic sprays to eliminate any pests.

3. Mmm, Mulch!
Mulch is a gardener's best friend. Mulch helps your plants retain moisture and adds extra insulation. During the winter months and long into spring and summer the mulch decomposes and adds rich organic matter to your soil. Applying a 1½-2” layer of mulch offers great protection. Inside a greenhouse, compost is a good mulch choice. While leaves and grass are great for outside mulch, they may harbor disease and your greenhouse environment must be a clean one. Coir makes a great greenhouse mulch; it is clean, holds moisture well and traps air. Trapped Air = Insulation.

4. Drink Up!
Just because your plants are not blooming or creating fruit for you, doesn’t mean they aren’t thirsty. While their water requirements diminish during the winter, the plants still need some water. Watering up to one inch per week is recommended if the greenhouse is warm and dry, but don't over-water; cool, moist environments harbor disease. Better to error on the side of under-watering.

5. Prune Early!
Too much change can be a bad thing. Make sure you prune early and at least 6 weeks before the expected first frost. How much should you prune? Cutting off any dead limbs or unhealthy growth is a good start. Some plants will thrive with a healthy prune while others may not fare as well. It is a good idea to do a little research before you start snipping away (for plant specific advise, check out gardening know how). However, be careful to not wait too long to prune, or your plants may not be able to harden off before winter. The shock of the cold could do some serious damage. This is also a great time to take cuttings and divide your plants.

6. Bundle Up!
Use frost shield bags and blankets to bundle up your plants (bed sheets also make good frost protectors). Just be careful if you use plastic sheets; they are not insulated so the cold it conducts may actually freeze the foliage and do more harm than good.

7. Heat Up!
Most plants are perfectly happy to overwinter well into the 30's, so an unheated greenhouse or cold frame is a great way to protect your plants from the elements. If you find your plants are actually growing you can lower the temperatures by letting in more cool air... just be careful to keep the soil temperature above freezing. In some areas a small heater is needed in order to regulate the heat at night. Make sure the greenhouse doesn’t get too warm during the day. Use your ventilation system to draw in the cool outside air to help regulate the temperature in your greenhouse to keep your plants dormant. Plants don't like to be locked up without fresh air, so even when it's cold out they still need some ventilation.

8. Where’s the Sun?
Most plants will tolerate a few months of lower light levels while overwintering. Keep your eyes open for yellowing or pale foliage, dropping or drooping leaves, and leggy growth. Any of these can mean that your plants need higher light levels. If needed, you can either relocate your cold frame or greenhouse to a sunnier location or supplement with grow lights. Some plants will completely die back and that is okay. They will still do well if they avoid a hard freeze and they benefit from and earlier and warmer spring when they are overwintered in the greenhouse.

9. The Great Outdoors!
Slow is the way to go in the spring, after the last frost has past. It is a good idea to bring your greenhouse temperatures up in the spring to get your plants ready to be moved outside. Approximate temperatures of 65° during the day and 55° at night will help your plants transition from winter weather to spring and prepare them for staying out all night. Once the last frost date has past, give your plants a few days in an intermediate zone (also called hardening off). You can set them under a covered porch or under a tree canopy to prevent sun scald before moving into direct, daily sun.

10. Oops!
It happens… some of your plants don’t look quite as happy as the others as spring nears. But don’t give up on them – they may return to life again. Prune back any dead tips or branches to just above new growth and wait and see.

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