Vegetable gardeners who want to extend their harvest season into late autumn and winter often rely on devices like cold frames, mini hoop tunnels and polytunnels. These structures are extremely effective, but even a simple – and inexpensive – blanket of leaf or straw mulch can be used to shelter root and stem crops, allowing them to be dug, harvested and enjoyed throughout the cold winter months.
This is a handy technique for urban gardeners or those with little room for a long term storage space like a root cellar. Instead, the crops are left in place in the garden and covered with a thick layer of insulating organic materials. Ideal candidates for mulching are root crops like carrots, parsnips, beets and celeriac, stem crops like celery, leeks and kohlrabi, and leafy crops like spinach and kale.
A side benefit to autumn and winter harvesting is that many cool and cold season vegetables taste better after they have been sweetened by frost.
Here are a few mulch basics:
To insulate root crops:
Before the ground freezes in late autumn, apply a layer of mulch that is at least one foot (30 cm) deep to the top of the garden bed. This ensures that the soil won’t freeze deeply and the crops remain harvestable throughout winter. Don’t panic if the top few centimeters of soil freezes beneath the mulch, as the roots will still slip from the soil with the help of a trusty garden fork or hand trowel.
After the mulch has been applied, top the bed with an old row cover or bed sheet. This will secure the leaves or straw in place and prevent them from blowing off the bed during winter storms. The fabric cover can be weighted down with rocks, logs or U-shaped garden staples (purchased from a garden supply store or made by cutting and bending foot-long pieces of wire coat hangers into six-inch [15 cm] long staples).
To insulate stem and leaf crops:
Stem and leaf vegetables like kale, cabbage, brussels sprouts, leeks and kohlrabi should be mulched so that the straw or leaves surround each plant. Again, cover the mulched bed with a lightweight row cover or an old bed sheet, but ensure that the cover doesn’t lie directly on top of the crop’s edible foliage (such as with kale). If the fabric freezes to the leaves of the plant, damage may occur.
Evergreen boughs can also be put to work to insulate stem and leaf crops. Kale and spinach respond particularly well to this season extending method. For mature plants, just cover the garden bed with branches and continue to harvest into the new year. For extra-early spring spinach, sow seeds in September, covering the garden bed with spruce or pine boughs in late autumn. By late March, the spinach will begin to re-sprout and be ready to harvest long before the traditional spring-seeded crop.
Three simple season extenders
Mulch is the quickest and most cost-effective way to stretch the harvest season for root, stem and certain leaf crops, but it doesn’t work for delicate salad greens like lettuce and arugula. Therefore, serious winter gardeners also rely on devices like row covers, cold frames and mini hoop tunnels.
Row Covers – A row cover may look like a flimsy piece of fabric, but this simple sheet can easily extend the autumn harvest by about a month. There are several grades of row covers – light, medium and heavyweight – with each offering varying degrees of frost protection. The covers can be laid directly on crops in spring and autumn, but for winter, they need to be ‘floated’ above the crops on hoops or other supports. If a cover is allowed to sit directly on winter vegetables, it will damage the foliage.
Cold Frames – A cold frame is a bottomless box with a translucent top that allows light to enter. The most basic type of cold frame is made of straw bales arranged in a rectangle and covered with an old window pane, shower door or a piece of clear polycarbonate material. Most gardeners build permanent wooden cold frames, using an old window or door for the top. With their low profile, ideal cold frame crops are salad greens like lettuce, spinach, Asian greens, baby kale and endive.
Mini Hoop Tunnels – A mini hoop tunnel is an easy-to-build structure that can be used to grow tall crops like kale, collards and leeks in winter, as well as to overwinter immature greens like spinach. The hoops are easily made from lengths of 10-foot (3 meter) long, ½-inch (1¼ cm) wide PVC electrical conduit that are bent into an upside-down U on the top of the garden bed. To secure the hoops, slip each end over 1-foot (30 cm) long rebar lengths that are pounded into the ground every three feet or so along the bed. For spring and fall protection, use a row cover to top the mini hoop tunnel, but for winter, you will need to add a layer of 6 mil greenhouse poly on top of the fabric. Secure your covers well with sandbags, rocks, logs or clips.
Niki Jabbour is the author of ‘The Year Round Vegetable Gardener‘ and the upcoming ‘Groundbreaking Food Gardens‘ (Dec 2013). She also hosts The Weekend Gardener, a weekly radio show airing Sundays from 11 am to 1 pm. Niki is also on Twitter – @NikiJabbour.