By Russell T. Nagata
University of Hawaii at Manoa
College of Tropical Agriculture and Human Resources
Komohana Research and Extension Center, Hilo
Here in Hawaii, where many of us truly believe that we garden in paradise, the idea of growing plants in a greenhouse or other protective structures appears to be a concept that is out of place. While some of us have greenhouse memories of the grand glass and cast iron conservatory greenhouses associated with botanical gardens and the ultra-rich of the industrial revolution era, greenhouses have a far longer history where people have a desire to grow plants under any condition.
The term greenhouse refers to houses in which the environment can be controlled in order to grow plants in an otherwise hostile environment. In short, a greenhouse improves the ability to keep insects and disease at bay, control light intensities, maintain a target temperature, and control moisture to maximize plant quality.
The concept of greenhouses began over two millennia ago in Rome, where the emperor’s desire to have cucumbers throughout the year needed a solution to bright summer sun and cold winter temperatures. The solution was to protect plants in structures covered with translucent panels of mica and other materials that allowed light to be transmitted to the protected growing space during cold winter days.
Plants were grown in containers and were moved in or out as needed. During hot summer months, plants were protected with cloth shading. As Europe emerged from the dark ages and began to explore the tropical world, they brought back plants and animals that required a tropical environment to survive. The development of greenhouses to fill this need slowly emerged. In the process, early horticulturist and botanist learned how to control the plant growing environment. These early greenhouse designs required lots of hands-on human intervention in order to maintain a suitable environment. Opening and closing windows and vents; moving plants in, out and around; stoking fires to warm the greenhouse were the job of many caretakers. In Asia, Korea in the 1400s had active greenhouses in which temperature and humidity could be controlled in order to grow mandarin oranges.
A key development contributing to the modern age of greenhouses and their wider availability was the ability to manufacture glass sheets in sufficient quantities and quality suitable for greenhouse construction. From the 17 century onward, greenhouses were built larger and more ornate, first in England and the Netherlands, then throughout the rest of Europe.
The main reason why we chose to grow plants in greenhouses and under cover in Hawaii is protection from insects, diseases, sun, rain, adverse temperatures and wind. One of the first large-scale uses of greenhouses in Hawaii was by the backyard orchid hobbyists, who built custom wood and glass houses in which to maintain their prized collections. If you look around as you travel around the island you will see many of these old greenhouses in yards of older neighborhoods.
In greenhouses you are able to control plant growth and time harvest by controlling light and temperature. You can screen out many of the unwanted pests that plague plant and fruit production. You are able to screen out fruit flies (oriental and Mediterranean fruit flies) which oviposit on many fruiting vegetables, butterflies and moths (pickleworm of cucurbits) who’s larva enjoy munching and boring through your plants and other insect pests.
As a side note, remember that if the plant needs pollination services from insects, screens will prevent pollinators from doing their job. Use self-pollinating plants, parthenocarpic setting fruits or manually pollinate the crop. On the windward side, greenhouses can also be used to protect plants from rain and excessive moisture that can lead to disease problems. Many diseases need free water to start the infection process going. A cover can protect the plants underneath from rain and dew reducing the amount of chemicals needed to otherwise control the problems when they develop. This is especially true for bacterial and fungal diseases.
When considering a greenhouse for your garden, important considerations are the intended use, size needed, cost, and zoning. Check with your planning department as an important part of the planning process. Today the home gardener can purchase greenhouse kits or build greenhouses from numerous designs and materials depending on cost and durability. Many of today’s greenhouses are built with polyethylene plastic sheeting or polycarbonate panels over a metal or wood frame, which greatly reduces the cost over glass structures.
In Hawaii one of the greatest problems of greenhouse production is the heat buildup inside of your greenhouse. Remember the term “greenhouse effect,” where heat builds up when it can’t escape. With the right greenhouse and a little practice you will be surprised in all of the different plants you can grow to beautify your home and to eat well.
For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the island. I can be reached at email@example.com.