All manner of bugs and slugs can find ways into your greenhouse. Whether you are using a small hobby greenhouse to start seedlings and protect tender plants from frost or are using a larger greenhouse on a commercial basis, it's easy to go organic. Getting to know your insect friends and foes is an important first step to controlling the critters that can do harm and encouraging the ones that eat the bad guys. Using organic fertilizers instead of chemicals to feed your plants is also an important aspect of growing organically in your greenhouse.
Identify the insect pest or pests you find eating your plants. Note their size, shape, color, general description and the type of damage they are causing. (See Tips.)
Spray plants that are affected by aphids, spider mites, mealy bugs and young "crawler" scale insects with a commercial insecticidal soap or make your own by mixing dishwashing liquid with water. Repeat daily until you see a significant reduction in the number of problematic insects. With organic gardening, you probably won't be able to completely eliminate all pests, but you can make an important difference in their population.
Deter ants, which introduce aphids and scale insects, by coating the stems of your plants with a product called "Tanglefoot." You can also smear this organic product at the base of wooden supports, plant stakes and even around the base of planter boxes.
Control adult "armored" scale insects by mixing 1 tbsp. of canola oil into your spray bottle containing insecticidal soap. You can also handpick these insects because most species are large enough to see with the naked eye. If you prefer, you can also purchase an ultra-fine oil, which also is effective against scale.
Snails and Slugs
Handpick snails and slugs by examining your greenhouse and plants at night.
Sprinkle diatomaceous earth on the soil around your tender plants.
Fill small jars half full of inexpensive beer. Dig small holes in the soil near your plants and then place the jars into them at an angle, so the lip of the jar is even with the soil. Empty the jars frequently (every 1 to 2 days, depending on how many creatures are inside) and replenish the beer at that time.
Spread iron phosphate granules (sold as "Sluggo") around the areas where slugs and snails might enter your greenhouse and around the base of the plants they eat. (Lettuce and other greens are especially appealing to these creatures, as are young seedlings.)
Spread a thick layer of compost around the base of your plants.
Make manure "tea" and water your plants with it. You can use chicken, rabbit or steer manure: Simply fill a bucket about one-third full with the manure, add water to fill it, and then allow it to "steep" for 1 to 2 days. Use it promptly. You can also spray the foliage of your plants with this nutritious liquid blend.
Purchase organically based plant food and use it according to label instructions.
About this Author
Barbara Fahs lives on Hawaii island, where she has created Hi‘iaka's Healing Herb Garden. Barbara wrote "Super Simple Guide to Creating Hawaiian Gardens," and has been a professional writer since 1984. She contributes to Big Island Weekly, Ke Ola magazine, GardenGuides.com and eHow.com. She earned her B.A. at UCSB and her M.A. from San Jose State University.