by Michelle Moore

The desire to expand ones plant portfolio is part of catching the gardening bug. Most gardeners try to cheat nature with at least one plant. Maybe with a little extra mulch will do the trick? Maybe there is a little extra space in the garage or even the house? For some it’s more than one plant that displaces the family from their dining room, or perhaps takes over every South facing window in the house. With the right space, nearly anything is possible but not always practical. While you may be able to achieve the most ambitious results, most of us prefer to push the boundaries without creating more inconvenience and cost. With a greenhouse it is possible to stretch the limits with nature’s help instead of fighting nature.

Over the years of visiting public and private gardens I’ve noticed a trend: behind nearly every great garden there is a greenhouse. The skill of the gardener, the design and careful amending of the soil combined with a controlled environment produces magical results. In some gardens the greenhouse is part of the design element; in others it’s the secret ingredient behind the scene. You don’t need to have a show piece garden to benefit from a greenhouse or season extender. You do need a greenhouse or at least a plan if you want to save money, to extend your harvest, or push the envelope a bit for that special plant.

In almost every climate there is something that can be grown in every season with the right tool.  With creativity and a little knowledge, the “Dead of Winter” can take on new meaning for you, as can the “Heat of Summer”. Improve upon any season you choose. Most people choose to shorten fall and winter, lengthen spring and summer. Skip one or two zones ahead with a little planning. If you have a greenhouse, you don’t have to add much heat or light to be the envy of the neighborhood.

Aids to Extend the Season

Greenhouse gardening is the easiest and most enjoyable way to save precious plants that can’t survive in your climate. If you have a small garden or don’t have the budget for a greenhouse there are still things you can do. If you plan to grow all year, focus on the plants that already take advantage of the season. Many plants require a cold period while others are warm loving. Some plants need only a little extra protection to make it through winter. For those hardier plants, mulch may add enough extra warmth to protect tender roots from frost. If tender foliage is the concern, a frost blanket will protect tender leaves and add a small amount of insulation. For plants in raised beds, consider a cloche or small cold frame over the bed. The added protection will raise the soil temperature as well as protect leaves from frost. For even more of a head start, add heating cables to the bed. Just a few degrees of additional soil heat may be enough to help transplants take root or small seeds to sprout. Cold frames and cloches work best in early spring before the warm days arrive. Beware that small structures overheat quickly and rarely provide ventilation mechanisms.

The techniques mentioned above may be necessary for plants remaining in your landscape. For plants in containers, there is no place like the greenhouse. The protected greenhouse environment is good for you too. Life in an otherwise barren landscape allures the senses. The promise of spring and hope for growth is enough to nourish any spirit, plant or human. In a greenhouse environment it is possible to control many factors. Not only will a greenhouse add heat to a cold night, a greenhouse serves many functions throughout the seasons. Greenhouses protect tender seedlings from torrential rains, hail and wind. Rodents, birds and other creatures don’t threaten the protected tender starts.

With the right greenhouse, heat, and ventilation it’s possible to create an oasis almost anywhere. Most of us don’t have an option to create a tropical jungle, so it’s important to understand the limitations. Most greenhouses provide heat and protection from the elements, but few protect against radical temperature swings. Rapid transition from warm to cold shocks plants more than just cold. Greenhouses covered with single layer material offer no insulation and will have drastic temperature swings. These structures work well for keeping frost at bay, protection from wind, and adding warmth during the day but it’s unlikely they will do more. Maintaining a more consistent temperature requires an insulated covering at a minimum.

In warmer months, overheating is a consideration. Clear coverings heat up quickly and require ventilation and shading. In some areas a greenhouse provides cooling and shelter from the sun’s burning rays. Greenhouses that provide cooling require diffuse light (a covering that is not clear), ventilation, fans, insulation, and some type of shading.

There are two basic approaches to overwintering. One is to place plants in the greenhouse and hope they survive. This approach sounds overly simplistic, but it works well if you have a large number of plants or if your stock is easily replaced. The other option is to mimic the plants native conditions as much as possible. This approach requires research and a bit of specialized attention like adding mulch to a specific plant inside a greenhouse, or using a frost blanket. Some tender plants may require some type of bottom heat such as a container wrapped in Christmas lights, a seedling heat mat, or added heat.

Learn what Plants Need to Grow

Growing successfully beyond your zone requires a basic understanding of a plant’s needs. All plants need 5 basic inputs to thrive: Light, heat, water, nutrients and carbon dioxide. With a greenhouse, you can more easily control the light, heat and water than you can outdoors. Plants in a greenhouse receive a different type of light than those outdoors. If the covering is clear see through, the light is magnified and can be more intense than natural light. If the covering is a milky white color, the light is diffuse, spreading the light to more areas. It is a misnomer that more light is better in a greenhouse. Most plants require roughly 70%-75% available light. More light means more heat, not more growth. As mentioned earlier, overheating is a major danger in clear greenhouses. A few ornamental flowering plants require intense direct light to flower. These plants are best kept in a greenhouse until conditions are favorable, then moved outside where they receive optimal light for flowering.

Nighttime temperatures are crucial to maintaining active growth. Overwintering and jumpstarting plants may require no heat at all, but actively growing is another animal. Plants absorb light during the day for photosynthesis. The food produced from photosynthesis fuels growth which happens at night. A warmer nighttime temperature results in faster growth. Controlling and maintaining a consistent temperature goes back to insulation. The more insulation the covering has, the greater the heat retention at night. Better heat retention means less added heat and more active growth. A greenhouse may be a solar furnace during the day, but if all the heat is lost an hour after the sun goes down, plants are in for a wild ride on a cold night.

Lastly, it is possible to maintain better water control in a greenhouse. Water evaporates more quickly in container but soil additives and conditioners will maintain moisture. It is easy to add humidity or vented excess humidity out of the greenhouse to create the desired climate.

Enjoy your garden and grow all year. Don’t be afraid to experiment; the worst thing that can happen is you lose a plant you shouldn’t be able to grow in the first place. As you gain experience and learn more about each plant, you will find ways to push the boundaries even more. Taking cues from the plants themselves, you’ll find new ways to keep them living longer. You’ll profit from longer harvests and most of all, you won’t have to restrict your gardening enjoyment to only a few months of the year. Take the plunge and surprise your friends with fresh leaf lettuce in December and tree ripened grapefruit in February.

Michelle Moore is the General Manager of the Greenhouse Catalog where she has worked with greenhouses for 20 years. Michelle writes and lectures about greenhouse gardening as an Oregon State University Master Gardener and member of Garden Writers Association. Michelle is a Fulbright Scholar and earned an International MBA from Thunderbird, School of Global Management. Michelle and her husband live in Oregon, but they enjoy hints of summer all year in their greenhouse! You can contact Michelle at: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it or http://www.greenhousecatalog.com/.