The Cold Frame, a Gardening Advantage

(Photo: Our cold frame in early spring)

Cold Frames

A cold frame, with a little care and effort, can extend your gardening season at both ends and offer three possible planting periods: fall plantings can be harvested in May, May plantings can be harvested in August, and August plantings can be harvested in November. Plant again in November for May. However, these planting periods depend a great deal on what is planted and on changes in the weather.

A standard cold frame is a do-it-yourself (DIY), above-ground or partially below-ground container which may be “floating” (isolated from ground soil if the ground soil has possible contaminants), or it can access the ground soil for unlimited depth.

Of course, there are small hard plastic or plexiglass cold frames available for purchase and if a 4×3 above-ground frame suits your needs, then try one of these as they come equipped with insulated walls and a heat triggered exhaust system.

A DIY wooden cold frame with a lattice frame cover can be fenced (e.g., chicken wire) to keep out critters, plastic covered (e.g., 6 mil construction plastic used in construction for basement vapour barriers) to control moisture and to increase the heat of the spring sun, fabric covered (e.g., row covers which let in rain and sun) to keep out bugs, and screened (e.g., sheets of window screen, recycled mosquito netting, netting borrowed from a canopy or pergola, or purchased screen from a garden supplier) to reduce the strength of the summer sun on cool weather vegetables (e.g., lettuces, spinach, kale, coriander). The lattice frame is needed to support the various covers, but a glass or plexiglass cover does not need the lattice frame support.

(Photo: recycled canopy frame hinged in the middle and the top + Chicken wire)

(Photo: 6 mil construction plastic — lasts for years!)

(Photo: purchased sun screen but any screen material will do to reduce sun strength)

The container, which has one low side and three high sides, should be aligned east to west with the low side facing the southern sun which in spring is at a low angle.

The size of the cold frame is up to you; however, be sure you can reach all corners. My current cold frame is three feet wide x six feet long and backs on a chain-link fence. At three feet wide, I can just reach the back of the container without climbing in, but I do have to lean in with hands on soil to work the back. Depending on what covers you use and where you place the cold frame, you may have 360 degrees of access, but generally in an urban, small space environment this is not the case (e.g., front access only if the cold frame backs on a fence or wall).

Any untreated, unpainted wood can be used, but spruce, pine, and fir (SPF) will last about five years. Eastern white cedar or hemlock will last from 10 to 15 years. The thickness of the wood is up to you. The ideal would be thick walls of 4×4 posts which offer more insulation than boards, will not warp as readily as boards, and do not need supports for the walls and corners. (I used 4x4s for my 4x6x2 ft. high box bed with 360-degree access – please see “Box Beds”)

For my 3×6 cold frame, considering cost, positioning, backyard room, and my desire to experiment, I chose 2×6 in. x 6 ft. rough cut eastern white cedar boards and instead of 2x4s to support the sides, I used galvanized metal plates on the outside to attach board to board and metal “L” brackets wrapped around the corners on the outside to hold the corners.

(Photo: 2 in x 6in boards of eastern white cedar & metal fasteners)

If you choose 2x4s to buttress the sides and ends, place them outside the container rather than inside. 2x4s on the inside will reduce space, get in the way of you working the soil, provide more nooks and crannies for insects and diseases, and will not hold as well as outside posts — boards should lean against the support posts rather than away from them. Support posts on the inside of a box allow the inside pressure of soil and water to push the boards outward away from the support posts and place undue pressure on the fasteners (nails or screws) until they fail – as they surely will in time – I have seen many failures from Winnipeg to Halifax; wet wood softens and will not hold the fasteners for long. Medieval European churches, ship builders, backyard above ground pools, and Canadian hockey rinks, all use outside supports to buttress the walls and to maximize the useable space inside.

Cold Frame Construction

First remove the grass on the chosen site. If you plan to sink the frame (recommended), dig a hole slightly larger than the dimensions of your cold frame-to-be, and shovel down 12 inches – a little deeper if you wish to provide more root room for larger plants. The lower section of the cold frame will be below ground for added insulation. Build the first 18 inches of the frame and place it on the site or in the hole (which leaves 6 inches above ground). Then add boards to the north side of the container as well as the east and west ends – more boards if you wish greater head room for larger plants. The container will now have a south-facing front of 18 inches and the other three sides of the frame will be 24 inches high or higher. Note, the height of the front determines the height of the soil level in the cold frame. Most cold frames I have seen taper the end walls from the high back, down to the low front so a single hard translucent or clear cover (glass, plexiglass, or plastic) will run from the back wall straight down to the low front. I left my side walls high to increase head room in the first 2.5 feet from the back, and then developed a double-hinged frame to cover the cold frame and support the different soft covers we planned to use. The double hinged frame lifts up and folds like a bi-fold door on top of the wood structure.

If you isolate the cold frame from ground soil, place a sandwich of landscape fabric, medium gravel, and landscape fabric in the bottom of the cold frame. The fabric will prevent the gravel from sinking into the ground soil, prevent your garden soil from sinking into the gravel, and prevent plants from reaching the ground soil.

(Photo: cold frame base + landscape fabric underlay – no underlay if plants access ground soil)

If there is no need to isolate the frame, then fill the cold frame with new garden soil and top dress with compost to 1 inch from the top of the front, south facing board. Do not bank the soil higher at the back wall; a flat soil bed is best for planting, watering, and fertilizing. You do not want water to flush all your seeds down to the front of the cold frame!

Cold Frame Covers

The top of the cold frame can be either glass, translucent hard plastic, or 6 mil construction plastic spread over a supporting frame strong enough to take the snow load. On my first cold frame, I used an old glass storm door to cover the top – this cold frame was of recycled red cedar (former picnic table) and the box was a rectangle without the low south side. It also had all support posts inside the box! The rectangular box worked well, but a low south side would have reduced the shadow of the south wall and that would have been better for light and heat from the spring sun. Supports outside would have made gardening easier when cultivating and would have increased the inside space. Eventually the fasteners failed, the boards pushed outward, and soil leaked out.

My present 3×6 cold frame uses 2 metal lattice panels recycled from a deck canopy to make a support frame hinged at the back of the cold frame and hinged again in the middle between panels to I could lift it and fold it like a wing/folding door used in some closets. This metal frame supports a variety of coverings: chicken wire, a 6-mil translucent plastic, a row cover, bug or bird netting, and a sun screen.

(Photo: Hinged frame)

Cold Frame Insulation

In winter, I surround the cold frame with insulating material. I have seen both hay bales and Styrofoam sheets used, but I do not recommend either one. For winter insulation, I surround our cold frame with bags of shredded leaves from the neighbourhood, and then use the shredded leaves as mulch the next spring throughout our in-ground garden plots. Worms love leaves, soil benefits from leaf decomposition, cats needing a toilet and squirrel gardeners hate them, and weeds cannot see the sun! Leaf shredding can be done with a gas or electric shredder, running a lawn mower over the leaves, or sucking leaves through a leaf blower/vacuum. I found the leaf/blower vacuum the most convenient method. If you use a lawn mower, be sure to add the grass catcher — raking shredded leaves is a thankless task and garbage bags often do not want to cooperate without a support stand. For me, it was easier to place the leaf-vacuum bag inside a garbage bag and shake the contents into the garbage bag.

(Photo: garbage bags of shredded leaves – more to come — as insulation)

As there is not a lot of head room in a cold frame, we plant lettuce, spinach, kale, and cilantro in the fall for a spring harvest. A sun screen helps when we wish to prolong the lettuce growing by reducing the light intensity and heat of the sun so the lettuce, spinach, and the cilantro will not bolt too quickly and become bitter to taste.

In spring, a cold frame can also be used to germinate seeds to be transplanted later in pots or plots, and during the summer, the top can be uncovered so the cold frame can be used for tall plants.

(Photo: cold frame converted to a topless container for tall summer plants)

Caution: cold frames heat up quickly and the heat must be controlled – ours reaches over 30 degrees C in April. Keep a thermometer in the cold frame and check it often for heat. Intermittent ventilation is essential, but you must catch the problem before it cooks your plants! One year I waited too long (e.g., checked at 1 p.m. instead of at 10 a.m.) and cooked everything. Luckily, I was able to nurture all the plants back to health, but it was three weeks of tender loving care, and still, it was touch and go for some.

Another challenge to be aware of is tree or shrub roots getting into your cold frame. Build away from trees and shrubs if possible and if not, put a fabric floor in the cold frame and be sure the fabric has a larger footprint than the cold frame. If there is a gap in the fabric or a hole, a tree will find it! A neighbour’s linden tree found mine! Trees and shrubs will send roots to wherever nourishment is best – well fertilized cold frame soil is a very nourishing plant restaurant, providing both rich food and water, thanks to you. The invading tree roots will become so thick you will not be able to work the soil.