Greenhouse growing allows you to enjoy fresh herbs and vegetables through the winter months, and it frees you from some of the problems you may encounter in the outdoor garden.
But greenhouses come with challenges of their own. Read on to learn how to deal with insects and disease problems and with plants that just won’t grow.
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Greenhouse Problems Caused By Pests
Many of the common insect pests that plague your outdoor garden can be kept out of a screened greenhouse. (This is especially true if your greenhouse plants are grown in containers over a solid floor, not in the ground.) For example, I can’t grow tatsoi in my outdoor garden because flea beetles devour it, but it thrives in my greenhouse.
However, beneficial insects, birds, toads, snakes, and other animals that eat pest bugs are also shut out, which creates opportunities for other pests. I don’t have issues with aphids and whiteflies in my outdoor garden, but some years they infest my greenhouse.
To keep the same problems from coming back year after year, I clean and heat-disinfect my greenhouse. I vacuum it thoroughly and then shut it up tight for at least three weeks of long hot summer days. This gets rid of many pest eggs (and also disease spores—see next section). Some growers scrub greenhouse surfaces with disinfectants instead of using heat treatment.
Greenhouse Pest Control: How to Tackle 5 Common Insect Problems
Here are a few common greenhouse pests. For more identifying information and drawings, see Greenhouse Insect Management, a publication from the University of Kentucky. Suggestions for controlling pests come from Rodale’s Garden Problem Solver and from my own experience.
Aphids are tiny soft-bodied insects that crawl on the undersides of leaves and suck sap. Pale dry spots on leaves, followed by yellowing and drying of whole leaves or plants, may be caused by aphids.
Two sprayings of Safer’s or another organic insecticidal soap will generally get rid of aphids. If that fails, one spraying of the organic insecticide pyrethrum should take care of the problem. Make sure you thoroughly coat the undersides of the leaves.
You can also reduce aphids by introducing ladybugs to your greenhouse. In my old wooden house ladybugs crawling out of the woodwork during the winter get to be a nuisance in some rooms. I relocate them to the greenhouse.
Caterpillars of many kinds chew large, jagged holes in leaves. If there are only a few holes (and caterpillars) you can pick caterpillars off by hand. Look for them near visible bite holes, or above their soft greenish droppings. A serious infestation can be controlled by spraying the organic insecticide Bacillus thuringiensis (Bt).
3. Fungus Gnats
Fungus gnats are tiny black flies that lay eggs under the soil. Larvae of some species feed on plant roots and stems and can destroy plants. Sprays of Bt or pyrethrum will control them.
Thrips are flying insects almost too tiny to see with the naked eye. Larvae and adults suck sap from flowers, leaves, and stems of greenhouse plants so leaves look scorched and flowers disintegrate. They can be controlled in the same way as aphids.
Whitefly larvae look very similar to aphids, though the small winged adults look different. As well as sucking the juice from plants, they excrete sticky “honeydew” on which black fungus often grows. Spray the undersides of leaves as you would for aphids.
To get rid of adult whiteflies and stop them from laying more eggs, Rodale’s Garden Problem Solver recommends making a sticky trap. A piece of cardboard or Masonite painted school-bus yellow and coated with petroleum jelly will catch many adults.
Problems Caused By Mildew, Mold, and Fungus
Various types of mold, mildew, and fungus can thrive in the warm, moist, still air of a greenhouse. Powdery mildew is a particular problem for greenhouse growers. Affected plants appear to be dusted with flour. Leaves dry out and turn yellow, and plants die.
Good ventilation will make plants less likely to mildew. Make sure that you have windows, doors, or vents on opposite ends of your greenhouse so air will flow through. You’ll probably also need a fan to keep the air circulating on windless days.
Look for disease-resistant plant varieties. I enjoy growing flat-leafed kales like Red or White Russian in my outdoor garden. But in my greenhouse, these kales tend to develop powdery mildew. Curly-leaved varieties like Winterbor and Vates resist disease better, so I rely on them for my greenhouse crop. Check with your local Cooperative Extension or with experienced gardeners about disease-resistant varieties of the crops you want to grow in your area.
One Cooperative Extension article also recommends keeping leaves up off the soil and separated from it by some sort of barrier. You could use mulch for this.
Organic fungicides can sometimes help to control powdery mildew and related diseases. I’ve tried Bacillus subtilis (Serenade) and also a homemade spray made of 1 part milk to 7 parts water. Outdoors these sprays sometimes slow the disease down. Inside they haven’t done much for me, but I’ve heard some growers report better success.
Plants That Just Won’t Grow
Sometimes you can’t see insect damage or disease, but your plants fail to thrive. Here are a few possibilities causes:
Temperature range—What is the preferred temperature range for the crop you’re growing? How warm is your greenhouse, by day and by night? Too much or too little heat can cause problems.
Basil or tomatoes, which like temps between 65 and 80 according to Rodale, will grow pale and stunted if they stay much cooler than that for too long. Lettuce and tatsoi may grow stunted and go to flower instead of producing large leaves if it’s too hot. See the section on heating and ventilation in our Types of Greenhouse article.
Nutrient requirements—If you’re growing plants in containers not in the ground, make sure they’re getting enough nutrients, either from the soil or from foliar feeding. 1/3 to ½ of the growing medium I use in my containers is compost. (In the 3×8’ grow box, the rest is sandy soil; in smaller pots, the rest is a blend of peat moss and perlite.) In the middle of the winter, I add fresh worm compost to the tops of the containers.
Fish emulsion and kelp extract are organic fertilizers you can spray onto leaves for a quick nutrient boost. Fish emulsion provides NPK, and kelp provides micronutrients. (Fish emulsion smells. If you have an attached greenhouse make sure you’ve shut it off from your house and opened it to the outdoors before, during, and for some time after, spraying.)
Water level—Plants can be stunted by too much or too little water. If you’re growing in containers, make sure they have good drainage holes. Use a growing medium that includes plenty of peat moss, perlite, sand, or vermiculite to encourage drainage—garden soil will clump up and can either waterlog or dry out.
The usual test for soil moisture, in the ground or in containers, goes like this: Pick up a fistful of your growing medium and squeeze it. If water comes out between your fingers, it’s too wet. Then open your hand. If the medium collapses into dust flakes instead of staying in a clump, it’s too dry. But some plants (like thyme) like an unusually dry growing environment. Know what each of your plant’s needs.
Light requirements—Even with water, light, and nutrients, plants won’t grow rapidly without adequate light. My deep-winter greenhouse crops—kale, chard, lettuce, tatsoi, and herbs—don’t get artificial light.
Since my greenhouse is attached to my house, it stays warm enough year-round. Nevertheless, the basil always dies around the winter solstice. The other herbs and greens don’t die, but they go into a sort of suspended animation, growing very slowly for a month or so before the lengthening days revive them.
I do use grow lights for my spring eggplant, pepper, and tomato seedlings on cloudy days. You can use grow lights year-round if the expense doesn’t bother you, and if you have a way to pull the lights back out of the way when the sun is shining.
The more controlled greenhouse environment removes some of the problems you face in outdoor gardening. It also adds some potential troubles of its own.
Each greenhouse setup will have its own challenges and benefits. Pay attention to what’s going on in your greenhouse. Keep records of what works well and what goes wrong, and of how your attempted solutions work. With a little time and care you’ll soon learn the system that works best for you.