Extending the Growing Season: Fall & Overwintering Crops

by Valerie Jean Rose, WSU Extension-Skagit County Master Gardener

Vintage radishes Vintage lettuces Vintage onions

Here are two big secrets of undercover gardeners: if you use a protective cover, you can grow and harvest delicious veggies all year. Enjoy crisp, fresh, homegrown salads even when it’s so cold outside the slugs are shivering! The other secret is: the best time to plant your fall and winter garden is not in the fall or winter – it’s in the height of summer! OK, that may not be a secret, but it is counter-intuitive to plant cabbage, beets, spinach and salad greens in August for harvest in autumn and winter. If you wait and plant in October, the waning sunlight and falling temperatures will leave seedlings too small to grow, let alone survive cold wind and rain.

Now is the time to plant fall spinach.
Look for fall and overwintering vegetable starts at your favorite nursery or co-op. Seeds that can still be planted in mid- to late August include collards (for winter and spring use), endive, kohlrabi, leaf lettuce, mustard greens, Oriental greens, specialty greens (like corn salad), overwintering onions, radicchio, radishes, scallions, spinach, and turnips. Be sure to select varieties specifically chosen for fall or overwintering gardens.

So the two key secrets are plant cold-hardy seeds or seedlings in summer and get your garden’s protective fall and winter coats ready for the inevitable change of weather.

Garden cloches FurneysBell-jars for the garden.
Protective covers for garden plants range from re-purposed plastic bottles to heated greenhouses. For centuries, European gardeners used bell-shaped glass structures, called bell-jars, to extend their growing season. You can get similar results by cutting the bottom from a used plastic soda bottle, or a plastic “clamshell” food container, and placing it over the plant. Make sure there’s room for the plant to grow, and keep the plant mulched and watered. But it takes a lot of plastic bottles to cover rows of spinach and cabbage plants will get too big for most re-purposed food containers. A floating row cover gives you flexibility and protects many plants at once.

Lettuces grown under a row cover.

Lettuces grown under
a row cover.

A floating row cover is a sheet of synthetic fabric laid over plants to ‘float’ on top. The fabric lets in air and water while keeping insects out, a great year-round pest control without using chemicals. For the fall & winter garden, use a thicker fabric, sometimes called a ‘frost blanket,’ to protect your tender veggies. Leave enough spare fabric to give your plants some growing room! Use fabric staples or mounded dirt along the fabric edges to keep it from floating into the neighbor’s garden.  Beware – most woven fabrics and polyethylene row covers only protect plants if the temperature is 28F or above, though some heavy materials have proven to protect plants down to 20F.  And with no frame supporting the row cover (remember, it’s floating,) plants can be crushed by snow.
Let’s investigate the next level of undercover gardening: the hoop house.

Playing with hoops.
A hoop house is a plastic-enclosed structure held up by arches or “hoops,” usually plastic pipe. Cover the frame with a large sheet of plastic, secure the plastic to the frame and the frame to the ground, and you have a hoop house! The hoops need to be high enough for future plant growth, but some hoop houses are tall enough to walk through and wide enough for several rows of crops. Such a hoop house is large enough to be a portable greenhouse – you can move it to different areas from season to season. If your garden includes raised beds framed with wide boards, it’s easy to attach hoops to the boards and turn your open raised bed into a well-drained raised hoop house.

Plants inside a hoop house are protected from frost, heavy rain, insects and wind. Whatever size you build, make sure it is well secured or the inevitable wind storm will turn it into a giant kite that won’t protect your plants.

Hagey Hoop House Photo 3 Hagey Hoop House Photo 1 Hagey Hoop House Photo 2

[Download plans to build this hoop house on our Plans page, here – Ed.]

For a more permanent, and wind-proof, season extender consider using a cold frame. This is basically a mini greenhouse, a frame with rigid panes of glass or plastic. The frame sits on the ground, so there’s no need to create a foundation. This is the classic project for using those old storm windows gathering dust in the garage – or ask your friends, someone will be happy to share their spares.

Frames can be made of wood, brick, cinder blocks or poured cement for a really permanent garden fixture. If you want all the supplies pre-cut and ready to assemble, cold frame kits are also available in garden supply stores and catalogs.

ColdframeThe cold frame relies on solar energy, that is, heat from the sun. If you don’t want to trust in the availability of sunny days, a cold frame can easily become a hotbed. Heated by soil-heating cables, steam-carrying pipes, or fresh, straw-filled manure, these additional elements are buried beneath the plant- rooting zone. With or without an extra heat source, a cold frame will be most effective in a southfacing sunny location. Do you remember where the sun shines in your yard during the autumn and winter? Does your favorite summer garden spot become a shady frost-zone? Remember, the sun rises and sets low across the southern horizon in a few months, so plan and plant accordingly.

Watering in the rain?
Any of these well-sealed growing environments will protect your garden from heavy rains, but they also stop gentle precipitation from reaching thirsty plants. All of these season-extenders require you to monitor soil moisture and become the ‘rainmaker’ providing water throughout the winter. Be sure the row cover, hoop house or cold frame is sitting on well-drained soil. If necessary, dig out the clay and replace it with gravel, then add a good soil/compost mix.

These season extenders trap heat, which is helpful as autumn gets colder or winter’s cold drags into spring. When the sun comes out, it may be time to pull back the row cover, pull the plastic from the hoop house or open the cold frame. Don’t forget to put the plants back under cover when the sun sets.

So plastic bottles, row covers and frost blankets, cold frames, hot frames and hoop houses, oh my. But there’s one more undercover garden structure we haven’t explored: next time we’ll investigate what’s growing in the greenhouse.
For more great gardening articles by Valerie Jean Rose, visit Skagit County Master Gardeners.

Photo credits:
Bell jars, furneysnursery.com
Lettuces grown under row covers, WSU Dept. Of Crop & Soil Sciences
Hagey Hoop House, John & Janice Hagey


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