Expert Greenhouse Growing Tips

Start With Simple Components. Most greenhouses are made from sheets of plastic stretched over a metal frame. You can buy a kit with all the essential components. Mine is a Paul Boers “gothic” style kit (the arches come to a peak at the top, which is better for shedding snow). I paid more up front to get 1 1/2-inch galvanized steel pipe, rather than 1-inch, for added strength. If your area gets any snow or heavy wind, this heavier pipe is a good investment.

I also recommend using 6-mil plastic that has been treated to resist ultraviolet breakdown. It’s readily available from any greenhouse supply, and the version I use is guaranteed for four years. I use two layers of plastic with a small, energy-efficient blower to inflate the space in-between. The inflated “bubble” increases the insulating value of the cover, sheds snow more readily, and resists “chatter” in the wind, resulting in better wear.

Protect Your Foundation. When I first put up my greenhouse, I installed a wooden foundation to attach the “channel lock” into which the plastic cover is secured. Because I avoid using chemically treated wood, I used 2-by-8 pine boards with several coats of linseed oil. That was a bad idea — the boards rotted out after five years. My solution was to install a single course of 4-inch hollow concrete block on a small poured footer, then lock a better grade 2-by-4 (sealed against moisture) onto the top of the block foundation using J-bolts pushed into wet concrete in the holes of the blocks. The channel lock is still screwed into wood, but the wood is never in contact with earth or rain. With applications of sealant as needed, it should last as long as I do.

Choose the Right Size. The larger the greenhouse, the better its ability to buffer temperature extremes. That’s because a larger greenhouse will have a larger amount of thermal mass — in the form of soil — that is warmed by the sun during the day, resulting in warmer nighttime temperatures. Another reason to choose a larger greenhouse is that you’ll find more and more things you want to do with it.

Most greenhouse kits come in stock sizes, so buying a larger stock size may make more sense than ordering a smaller custom size. My initial plan was to erect a 20-by-32-foot greenhouse, but I found that by purchasing the 20-by-48 stock size, I picked up 50 percent more growing space for a mere $100.

Provide Adequate Ventilation. If you plan to grow vegetable crops in the greenhouse during the summer, your greenhouse will need heavy-duty exhaust fans and roll-up sides. For most winter growing, however, exhaust fans are superfluous. I know growers with 20-by-96-foot greenhouses who report that winter ventilation is adequate just by leaving the doors open as needed.

Brace for Winter. If you don’t remember anything else from this article, remember this: If you live in an area that gets snow, brace the hell out of your greenhouse! Snow is so light and fluffy, it’s hard to imagine the load it puts on a big structure as it accumulates, but the results can be dire. In February 2003, an unexpected snowstorm crushed my beloved greenhouse leaving inch-and-a-half steel pipes on the ground like spaghetti. After that sad experience, I put up 2-by-4 vertical braces to add support to the framing from the earliest possible date for snow in late fall until the latest possible date in early spring.

Care for the Greenhouse Soil

Each winter, I grow plots of tightly spaced forage crops to cut for my poultry (usually grain grasses and mixed crucifers), which I rotate over the greenhouse beds. As the spent root systems decompose, they increase tilth, fertility and humus. Of course, using compost in the greenhouse is also a good idea — it will help boost the microbial populations in the soil. Mulches have benefits, too. They will moderate the temperature in the soil, conserve moisture and decompose over time to increase fertility.

There are advantages to leaving the greenhouse soil fallow over the summer: The soil “solarizes” in the intense heat, which burns off soil pathogens and will desiccate even the most die-hard slug. Last summer, however, I realized I wasn’t doing anything to improve the soil in my greenhouse that was equivalent to my practice of cover cropping in the garden. So I grew a cover crop of cowpeas, which do well in the concentrated heat and the drier soil of the summer greenhouse. The project required a lot of water, but made a big difference in soil quality.

One caution: Avoid overfertilizing with nitrogen. Green leafy crops can accumulate unhealthy levels of nitrates, especially in the low light conditions of a winter greenhouse. I never add nitrogen fertilizers in the greenhouse, and I always use plant-based rather than manure-based (higher in nitrogen) composts. I am even concerned about the nitrogen added to the soil by my summer cowpea cover crop, and plan to follow it with a quick mixed grain cover to “sop up” some of that excess nitrogen before I plant other crops.

Bring the Chickens Into the Greenhouse

I’ve been intrigued for years by the idea of keeping chickens in the greenhouse, and I put it into practice in 2005. We installed 4-inch block, two courses deep, to make partitions for two chicken pens, enclosing them with poultry wire over light wood framing.

There are three benefits derived from keeping chickens in the greenhouse in winter. First, the CO₂ from the poultry’s exhalations promotes plant growth. (Before you think I’ve gone off my rocker factoring in chicken breath, consider that in the Netherlands growers pay good money for bottled CO₂ to pump into their greenhouses.)

Another benefit is that the body heat of the flock moderates the overnight chill in the greenhouse. I cannot test this proposition scientifically, but I had 43 chickens, three ducks and two African geese in there at the height of last winter — more than 250 pounds of warm living bird has to make a difference.

Finally, I hate confining my flock in the chicken coop during the winter. Now instead of doing that, I heavily mulch the garden area outside the greenhouse, enclose it with electronet fencing and release the birds onto it during the day. This has many benefits: The mulch protects the soil over the winter, the birds eat earthworms and slugs that live under the mulch, and the cold-hardy poultry enjoy the sun, fresh air and exercise instead of being confined to the boredom and stress of a coop.

Worm Bin: Out-of-Sight Earthworms

I experimented with a 3-by-4-foot worm bin for several years. The greenhouse renovation seemed like the perfect time to step up to more serious vermicomposting. We dug in two courses of 4-inch hollow concrete block for the bins, 40 feet right down the center of the greenhouse (see the photos in the Image Gallery). Every 8 feet, we put a cross wall of block, to create five 4-by-8-foot bins, 16 inches deep, each with two 4-by-4-foot lids made of three-fourths inch plywood on 2-by-4 framing. When the flock is out of the greenhouse over the summer, their two 8-by-8 pens are used for an additional 128 square feet of vermicomposting bin.

Because I needed access down the center anyway, I didn’t lose much growing space to the new worm bins. And those substantial 4-by-4 lids over the bins have been a godsend. I routinely roll a fully loaded wheelbarrow over them. They are a great place to lay out work projects. We’ve even set up a table on them and had a picnic there; and I addressed a seated class of 16 on them one raw March day.

Of course all the kitchen throw-offs that don’t go to the flock get fed to the worms. But my operation is way beyond the “worms eat my garbage” scale. I haul in pony poop by the pickup load from a neighbor who breeds and boards horses. The worms convert the manure into castings (earthworm poop), one of the best of all natural fertilizers. Last winter, the populations were finally high enough to make regular harvests of earthworms to feed the flock, a nutrient-dense, high-protein addition to their diet.

Winter Gardening Strategy

I don’t add any artificial heat to my greenhouse, because I don’t think using heat from fossil fuels to grow tomatoes in January is a sustainable practice. Instead, I choose naturally frost-resistant plants and count on the structure of the greenhouse itself to protect them from winter’s extremes. You could say my greenhouse gardening strategy is to imitate an unusually mild winter, not to teleport my plants to the tropics. The greenhouse protects plants from winter extremes not only by slowing temperature changes but also by keeping wind and cold rains at bay.

Plants such as lettuce are not bothered much by freezing air temperatures. They have learned a neat little trick to survive, which is unsettling the first time you see it: As air temperature drops, the plants move water out of their cells into the intercellular spaces, so that freezing doesn’t disrupt the cell walls. The leaves go limp (and the frantic gardener assumes the crop is lost) — but then they perk back up as the sunlight warms the greenhouse and the cells rehydrate.

The more critical factor is to prevent freezing deep into the root zone — this is the key to successful winter gardening. You could think of the soil inside the greenhouse as a rechargeable battery. During the day, it charges from the heat energy of the incoming sunlight. At night, it quickly loses that stored energy, but it has a huge amount of heat to lose before the soil starts to freeze. I’ve gone into my greenhouse after a 10 degree night to find only a quarter-inch of frost on the surface of the growing beds. Before the frost gets deeper into the root zone, the new day begins the cycle again.

Greenhouses: The Mirror Season

Experienced gardeners might have some difficulty adjusting to the paradoxes of winter gardening. We have to relearn many of our assumptions, particularly about scheduling our crops. Unlike in spring, when the season is opening out into greater warmth and longer days, in the fall it is shutting down into increasing darkness and deeper cold. The biggest challenge will likely be the shorter day length, rather than the lower temperatures.

The bad news: During the darkest time of winter, there is insufficient solar energy to support vigorous growth. If you start your plants too late to accomplish most of their growth before the short days, they will survive the cold temperatures, but instead of growing actively, they will sit and sulk, awaiting sunnier days.

The good news: On the other hand, if you get the timing right, you can produce, say, a mature head of lettuce before the darkest days and it will stay fresh much longer than in the summer. That perfect head of lettuce that would spoil within a matter of days in June will stay in prime condition for two or even three months in the middle of winter.

When you start your crops in the late summer or early fall, start far more than you think you will need. As you harvest, you will not be able to start new crops, but if you have plenty “in the bank” at that point, you can continue making generous harvests until longer days make possible some late-winter crops.

Greenhouse Crops

I start almost all my greenhouse crops as seeds under my growlights in the basement, then move them into the greenhouse when they get too big for the growbench. The greenhouse is simply too hot for direct sowing in late summer and early fall, when most winter crops need to be started. These are some of my favorite greenhouse crops.

Salads. Lettuces are quite resistant to frost, though not as cold hardy as some other winter garden plants. I grow a dozen different varieties — I’m easily bored with only one or two textures, colors and tastes.

Chicories are my favorite winter salad. If you’ve been turned off to stringy, bitter endives and escaroles from the supermarket, be assured that — in the chill and reduced light of the winter greenhouse — chicory’s bitterness is tinged with sweet, and the stringy toughness is replaced by a delightful juicy crunch. (An unusually good source for chicory seeds is Seeds from Italy.)

Lesser-known salads include mâche and edible chrysanthemum. Some are astoundingly cold hardy, such as claytonia (or miner’s lettuce) and minutina (Herba stella). And don’t forget scallions as an easily grown addition to winter salads.

Cooking Greens. Spinach is extremely cold hardy. I make several sowings during the winter growing season. I also plant crucifers — including mustards, raab, Oriental greens such as pak choi and tatsoi — all are tasty and nutritious “potherbs,” or cooking greens. Chard (or Swiss chard) is a type of beet bred for its large tender leaves and rapid re-growth, rather than its roots. It is cold hardy and productive.

Green onion and garlic tops also make great cooking greens. When I plant my garlic crop in the garden in the fall, I set aside the smaller cloves for growing “garlic scallions” in the greenhouse. We also sort out the smaller stored onions, or the ones that have begun sprouting, and plant them in the greenhouse for their beautiful and nutritious green tops.

Brassicas that head (such as cabbages and broccoli) are more likely to develop large, tight heads if grown in the late-winter greenhouse rather than in the fall. Loose leafed kale, however, is an excellent crop for the fall-winter greenhouse if you start your transplants early enough.

Get an Early Start. Root crops such as beets or carrots are not suitable for planting in the fall greenhouse — they will grow, but do not receive sufficient energy in the shortening days to “make root.” I have, however, had excellent results growing carrots, beets, potatoes and daikon (as well as the smaller radishes) in late winter, and harvesting these crops up to two months earlier than their siblings in the garden.

I also use the greenhouse to give an early start to tomatoes, peppers and eggplants. I start them under grow lights as early as mid-January to the end of February, then move them to the greenhouse once they’ve started growing. I move the fastest-growing plants outside when the season has advanced enough, and harvest ripe fruits a month early. (A few tomato plants grown in the greenhouse itself offer those first vine-ripened tomatoes even earlier.)

Grow Green Forage for Livestock. To offer fresh green forage in the winter (which is beneficial even in small amounts), we reserve some of the greenhouse for growing “green chop” for our goats, poultry and rabbits. Grain grasses (wheat, barley, rye and oats), mixed crucifers (rape, mustards and turnips grown for the leaf), and peas are all excellent candidates for cut-and-come-again green forage.

More Greenhouse Gardening Techniques

Know When to Water. It’s best to water deeply from time to time in lieu of frequent shallow waterings. Water in the morning, as soon as the frost is off the leaves, to give the plants plenty of time to dry before temperatures fall at night. Avoid overwatering, which makes plants “sappy,” less able to withstand cold and other stresses, and less flavorful and nutritious as well. Test the soil with your finger: As long as you feel good moisture half an inch deep or so, it’s better not to water.

Encourage Natural Ventilation. A closed greenhouse gets surprisingly hot on a sunny day, even if the temperature outside is quite cold. Don’t stress your plants by leaving the doors to the greenhouse closed when it’s sunny. I typically shut my two large doors (one at either end) at night, then open them wide during the day. If the day is unusually cold, blustery and cloudy, I prop the doors partially open, ensuring air movement through the greenhouse during the day. Good ventilation is important for disease prevention as well.

Balance Your Insects. My general approach to leaf-eating insects is not so much about control, as it is about balance. Therefore, I encourage all the flowering plants I can inside the greenhouse. These flowers provide pollen and nectar that attract lacewings, ladybugs and other beneficial insects. For example, the yarrow I planted last year bloomed early in spring, which encouraged the lady beetle population, so I had far less trouble with aphids this spring. Beneficial insects seem to migrate out of the greenhouse into the garden as it starts to bloom, boosting insect diversity there.

Enjoy the Heat! Last but not least, a greenhouse is the perfect cure for the wintertime blahs. It may be 20 degrees with a rude wind blowing over packed snow outside — as long as the sun is shining, you can step into the greenhouse and it’s Miami!

A Hot Box for Super-Early Spring Plants

In late winter (or early spring) I make a “hot box” for tender transplants. I place gallon jugs and 5-gallon buckets filled with water into an emptied worm bin, where they are a foot or two lower than the surrounding plant beds. Then I lay scrap plywood over the tops of the buckets. This creates a huge heat sink during the day, and the air temperature over the buckets at night remains many degrees above the air in the rest of the greenhouse. I added a light wood frame to drape scrap blankets over the hot box, cozying them even further. Jack Frost is never going to touch my babies!