Greenhouse Growing for Increased Productivity
By Emily Skelton and Scott Vlaun

A few flats of onions sprouting in the greenhouse on a cold February day is a sure sign that the end of winter is in sight. But March and April are when the sowing of seeds begins in earnest and the greenhouse comes to life with a diverse array of annual and perennial seedlings. In colder climates, early spring is also a great time to start growing salad greens in containers or permanent beds in the greenhouse. Successfully starting all types of seeds in the greenhouse is richly satisfying and the mark of an accomplished gardener. What could be more rewarding than witnessing the mystery of a tiny seed growing into a huge cabbage, broccoli, or squash?

Start Early for a Colorful and Bountiful Summer Garden
Annual flowers that can potentially bloom all summer, such as marigolds, zinnias, and calendula, can achieve a much longer bloom period if plants are started indoors four to eight weeks before your last frost date. This is especially important for long-season varieties such as Belles of Ireland or Everlasting Starflower. Some perennial flowers such as our Carnival Hollyhock or Mixed Giant Dahlia, which normally take two years to bloom when grown from seed, can bloom in their first year if the seeds are started early enough indoors. Your seed packs will contain information about the optimal time for starting seeds in relation to your last frost date.

Many common vegetable varieties such as peppers, tomatoes, and eggplants are typically started in the greenhouse. In warmer areas where you might be able to grow tomatoes from seed in the garden, starting seedlings in the greenhouse saves valuable garden space and increases production. While most plant varieties can be started indoors and transplanted, there are certain plants, such as carrots, that must be direct seeded.

Even an unheated greenhouse is a great place to extend your season for growing cold-hardy greens like spinach, mustards, and kale. Add some heat and lights, and you can have fresh vegetables and flowers all year long, regardless of your climate.

A Greenhouse for Every Budget
A greenhouse can be anything from a custom-built addition to your existing home--involving financing, permits, and contractors--to simply fixing some greenhouse plastic over a frame of metal, wood, or plastic pipe. If you don't have the time or inclination for another do-it-yourself project, Seeds of Change currently offers two lines of greenhouses to satisfy most budgets.

For the economy minded or space challenged, our Collapsible Garden House provides 64 square feet of growing space that you can erect in minutes. It has great ventilation and is easy to store when not in use. You can even zip together 2 or more for extra space.

Our Wood Frame Greenhouses provide a more permanent and efficient space with insulated polycarbonate panels and automatic vent openers. Redwood construction (from sustainably harvested trees) makes them beautiful, durable, and rot resistant. Sizes range from the compact 6x8 foot to the luxurious 8x16 foot.

Getting Situated
Whatever style of greenhouse you choose, the first thing to do is to decide where your greenhouse will be situated in your garden. Sunlight, wind, and local regulations may all play a role in your decision. Across most of the colder northern U.S., you'll want the broad side of the greenhouse to face the low winter sun. In New Mexico, where we have much more daylight in December than the far Northeast or Northwest, we generally angle our freestanding greenhouses from north to south to capture the morning and afternoon light. Attached greenhouses will still be most effective facing South for maximum solar gain in the winter but need to utilize thermal mass, well-designed shading, and ventilation systems to avoid overheating in warmer weather. Careful observation of the sun's path across your yard will help you to orient your greenhouse for the maximum exposure.

Adequate air circulation is also important in the greenhouse, especially as the weather warms up in the spring. If the front door opens toward the prevailing winds, then you may be able to reduce the added expense of a fan. Create a cross draft by opening the door and a window at the far end. Depending on the size of your greenhouse, one or more vents high in the ceiling will help to prevent overheating.

A concrete foundation can be durable and easy to clean but can also be a problem if you want to move your greenhouse in the future or want the option of planting directly into the ground. If you do opt for a concrete floor be sure to provide adequate drainage and slope the floor towards it. We use a thick (up to one foot) layer of crushed stone in our greenhouses for its thermal mass and drainage. If you choose this route, you may want to lay down a weed barrier before placing the gravel. In very cold areas, consider digging a two- to three-foot-deep trench around the perimeter of your greenhouse and burying some rigid foam insulation to help prevent the soil inside from freezing. This is especially important if you will be planting in the ground and will also help reduce your overall heating requirements.

Before beginning any greenhouse project, check the local regulations and restrictions in terms of permits, building codes, potential underground plumbing, and electrical lines.

Design the Space, Control the Environment
Consider your use of the space. Do you want to be growing year round or just using your greenhouse for starting plants in the spring? Or will you be using the space for both activities? If you are using the greenhouse for both activities, make sure to leave enough room to move around, do your potting, and still have adequate air circulation and light when plants get big. Without proper ventilation and air circulation, plants are more susceptible to disease and pest problems. At least two automatic vent openers, one high in the ceiling and one low on the north or east side will help prevent overheating and increase air circulation. In the warmer months, a mesh shade cloth over the outside of the greenhouse can lower the temperature as much as 20 degrees. If you are plagued by common greenhouse pests, such as whiteflies or aphids, a timely release of beneficial insects can help control them.

The potting bench or propagation area is a space for starting seeds and cuttings, or repotting container plants. This can be inside the greenhouse if you have room, or outside in a small shed or other shaded area. The latter creates a cooler workspace when it begins to warm up in the spring, and it avoids taking up valuable growing area in the greenhouse. If you choose to have your potting area inside the greenhouse, locate it on the north side of the building so that it won't block winter sunlight.

Adequate shelving is important for keeping tools and supplies handy and off of your growing benches. Hinged or other removable shelving is a good option because you can use the shelf for the initial part of the season--even if it is over a growing bench--and then tuck it away as plants get bigger and need more room.

Light
In areas where winters are cloudy or days are short, it is important to maximize light in your greenhouse. You can increase light levels by painting interior walls white to reflect light back into the greenhouse space. Placing white gravel on the ground outside and/or inside the greenhouse, achieves the same effect as a small pond on the south side. Be sure there is nothing blocking the sunlight on the south side of your greenhouse.

Depending on your needs and the length of your days, you can supplement the natural light with electric lights. In remote areas, these can be powered by solar panels. For maximum growth, place your lights on a timer and have them go on one to two hours before dusk and stay on long enough to achieve about eighteen continuous hours of total light per day. For flowering and fruiting plants, the photoperiod should be reduced to around twelve hours per day. Don't wait until after dark to turn on the lights because plants begin to shut down photosynthetic processes approximately forty-five minutes before sundown. And, don't leave your lights on all night. Plants need a period of dark for respiration to occur.

For all you need to know about supplemental lighting (and pretty much anything about greenhouses), consult Shane Smith's excellent book Greenhouse Grower's Companion.

Photo caption: (top) Our 8x16 foot woodframe greenhouse at the Maine Trial Garden. (bottom) Early spring salad in the greenhouse.