Authored by: James W. Brown

Changes in seasons bring about changes in the external and internal environments of the greenhouse. These changes compel the grower to view things inside and outside the greenhouse differently. Although the idea is to attempt to keep the environment inside the greenhouse uniform, the way to accomplish this will change from one season to the next. Things that were previously outside may suddenly want to migrate indoors. You or your friends may even want to move some outside plants into the greenhouse.

Although the goal is to keep the environment in the greenhouse so that the plants are comfortable, have the things they need and are able to grow, the way it is done and the extent to which the goal is achieved will change with different seasons and outside environmental conditions. As the outside temperature increases to the plant comfort range, more air is moved through the greenhouse for cooling. Fungus spores and small insects can ride along with the air being drawn into the greenhouse. However, they all don’t ride out of the greenhouse with the out-going air. Some stay and set up housekeeping at the expense of your plants – and you.

Some summer outside dwellers look for a warmer environment for the late fall, winter and early spring. Rodents, like mice, are among such creatures. In different areas there will be different potential intruders. Major crops in some areas are harvested during a short period of time. This can leave large insect populations without lunch on very short notice. Your greenhouse could be one of the few locations providing the viable green plants they seek. Some of those many insects may find your greenhouse and like it, electing to stay. Keeping them out may be a challenge that requires extra preventative measures.

Prevention Through Exclusion

The first objective with respect to hobby greenhouse problems is to keep problematic creatures out of the greenhouse to begin with. It makes sense to go to great lengths to ensure greenhouse pest control within a hobby greenhouse – it can be very expensive to clean up or eliminate a problem once it has become established!

An area covered with fine insect screen just outside the air intake is one measure to consider. In most locations, it should be a screen fine enough to keep out Thrips. Because such a fine screen will restrict air flow, the screen area should be at least five times the area of the greenhouse air intake (see photo). Often this is accomplished by extending the greenhouse structure with a section that is covered with the screening material. Plastic sheeting can be installed over top of the screening material in the winter in regions where snow may accumulate on any horizontal part of the screened area. Unfortunately, screening does not keep everything out forever. However, it can – and usually does – slow things down enough to make it worth the investment.

Screening one end while at the same time letting in anything and everything as you open and close the front door doesn’t make much sense. If you really don’t think about what you’re doing, that is exactly what can happen. Some suggestions on how to avoid this, accompanied with the reasoning behind the suggestion, will be given below. Some may be harder than others to follow.

We suggest that you start all your plants from seed in your own greenhouse. By doing this, you will greatly minimize the chances of introducing unwanted organisms to your greenhouse environment. Realistically, though, starting all your own plants doesn’t always work. Some plants grown from seed don’t come true to type, and are typically reproduced vegetatively. When the plants you plan to grow are vegetatively reproduced plants, the use of stock that has been reproduced by tissue culture is one of the best ways to minimize the chances of introducing unwanted things to your greenhouse. Plants started by tissue culture are going to be more expensive than comparable plants started by more conventional vegetative reproduction techniques. The extra cost, however, will be small compared to the cost of treating an entire greenhouse for imported problems. Often tissue culture produced plants are not as readily available as conventionally propagated plants. They may need to be ordered through specialty sources and are less likely to be available for impulse purchases.

Next, the media used for seed germination and the media used for growing in a greenhouse should be commercially available, soilless media. This would include perlite, rockwool, and various soilless mixes. The purpose for this is to exclude insect and microbial pests that can live in soil. Although soil can be treated with either heat or chemicals to kill any undesirable inhabitants, many would consider either type of treatment environmentally unfriendly. For example, heat treatment kills at least some of the beneficial organisms in the soil, and it can also oxidize some of the soil organic matter.

Plants from outside should not be allowed to over winter in the greenhouse. They can bring in unwanted pests with them. Many greenhouse owners have given refuge to plants without realizing the possibility that they were introducing problems as they brought in their plants. Others have the thought occur to them a day or so later, sometimes spurred by the first evidence that a problem has appeared. You need to establish a policy regarding this in advance of the season when plants need to be brought in. Often, friends will request refuge in your greenhouse for a favorite plant or two. Having an answer ready for them – along with its reason – can save you from costly greenhouse problems.

Pets like dogs and cats are factors in potential greenhouse problems. This may well be a touchy area for some pet-owning greenhouse growers. The intention of the following advice is to help you minimize or eliminate the introduction of pests to the greenhouse environment. Not all pets need to be excluded from the greenhouse.

Outside dogs should not be allowed into the greenhouse any more than they would be let into your dwelling. There are too many things they could carry into the greenhouse. House dogs that romp through the yard, garden and adjacent fields on the way to the greenhouse should not be allowed into the greenhouse. Lap dogs, dogs that are carried or follow you on the path to the greenhouse are unlikely to introduce anything harmful to the greenhouse environment.

Cats are another consideration. There are different opinions about letting cats into the greenhouse. If the cat spends much of its time roaming around outside, it should stay out of the greenhouse. If it is primarily a house cat, there is less concern about letting it into the greenhouse. Some people, including some commercial greenhouse growers, have a greenhouse cat. The cat lives in the greenhouse. That is where it is fed and has its litter box. A greenhouse cat who is a good hunter can help with greenhouse rodent control!

Sometimes insects, mites or diseases can be taken into the greenhouse on materials or items that have been exposed to plants or produce in the grocery store. Used produce shipping boxes and plant shipping boxes should never be taken into the greenhouse. Even boxes that you have used to deliver your own greenhouse products should never be taken back into your greenhouse. They may have been exposed to other plants or infested produce in a storage area in the store. They could have picked up a common plant disease or mites that would then be introduced to your greenhouse environment.

Even you and your visitors could introduce problem instigators into the greenhouse! You and they should not enter the greenhouse after being in another greenhouse, a garden or an agricultural field. Even visiting your greenhouse after preparing vegetables for a meal in your kitchen could introduce a problem to your greenhouse. Washing your hands before going to the greenhouse should be considered standard procedure after working with any plant or produce in the house or on your job. In fact, depending on the extent of the potential contamination to which you’ve been exposed, it’s advisable to do anything from washing your hands to changing your clothing, including your footwear, before entering your greenhouse.

Some greenhouse operators, including some of the big commercial growers, have a foot bath through which people entering the greenhouse step. However, most solutions that can be used in such foot baths are unstable and break down within minutes. A foot bath with a three-day-old solution gives no physical protection to your greenhouse.

Common Plant Diseases

Plants have natural resistance to many diseases. Some of those resistances are inherent within the plant while others are provided to the plant through symbiotic relationships between the plant and a microorganism. The microorganism is usually a fungus and it usually lives within the soil. Microorganisms can be introduced into the soilless production system and are used to help in common plant disease prevention and elimination within the greenhouse.

Common plant diseases result from the intrusion of one of three main types of organisms. there are a few instances where the causal agent is not within the three categories, most common plant  diseases encountered by the hobbyist will be caused by an organism fitting into one of the three categories. Physiological and even nutritional problems can be thought by the inexperienced to be a disease.

Most disease-causing organisms can be classified within the fungus, bacterial or viral categories. There are some generalities and similarities within each area. We will look at some of these in general terms.

Fungus diseases are typically spread by spores. A fungus spore is usually a reproductive body featuring a relatively small size and the capability of getting the fungus started in a new location. It is often very readily spread by moving air or within flowing or splashing water.

It’s fairly simple to minimize the possibility of water-borne spores in the greenhouse by using municipal water or well water. River water, pond water or reservoir water is more likely to carry water borne fungus spores. Municipal water from such sources is usually treated, so common plant disease spores do not survive. If you use untreated water from rivers, ponds or reservoirs, you could introduce water borne fungus spores to your greenhouse. Such incoming water can be processed with ultra violet light water treatment systems to eliminate the risk of the introduction of fungus spores.

Air is blowing or being drawn through the greenhouse on a frequent basis during the warm months of the year. During these times, it is impractical to expect to exclude all airborne fungal spores from the greenhouse. The more realistic approach is to take steps to interfere with spore growth and development and its intrusion into the plants. This is achieved through environmental management.

Most airborne plant fungus spores need free water on the plant leaves or at least very high relative humidity in order to germinate and have a chance to gain entry to the plant. If high relative humidity could be avoided in the greenhouse, there would be virtually no fungus disease problems. In desert areas there are few or no problems with fungus diseases because the relative humidity can be kept down all times of the day for at least most of the year.

Most air borne fungus spores need several hours of free water on the plant leaves before they can germinate and gain entry to the leaf tissue. When the day temperature of the greenhouse is allowed to drop a few degrees to the night temperature, the dew point may be reached. At dew point, the air has more water than it can hold at the new, lower temperature. The water condenses out of the air on cool surfaces in the greenhouse, including the plant leaves and stems. Suddenly, the formerly dry plant now has an ideal environment for the fungus spores that have landed on it. The spores take advantage of the conditions and germinate. If the leaf remains wet long enough, the greenhouse growing fungus will find a stomata or a small wound in the leaf where it will be able to enter the plant. If the greenhouse growing fungus gets inside the leaf before the leaf dries off and the fungus dries out, the disease is then established in the leaf.

Some oxidizers, like hydrogen peroxide-based fungicides, can kill spores on the leaves before they germinate. Most conventional fungicides kill germinated spores before they gain entry to the plant. That, at least, is the objective. Once the fungus has gained entry into the plant, the process of eliminating it becomes a very complex and involved battle.

Bacterial Diseases

Plant diseases caused by bacteria are less common in the greenhouse than those caused by fungi. Bacteria are generally spread by contact. Therefore, a bacteria that gets into the greenhouse rides in on plant material or possibly people or pets.

Starting plants in the greenhouse from seeds, where practical, is a good way to minimize the possibility of bacterial diseases in the greenhouse. Established plants, bedding plants and cuttings can transmit bacterial diseases.

Some bacterial diseases are seed-borne. They can live on or in the seed while it is being stored. Most seeds are treated so that any bacterial disease that could be spread by the seed is very unlikely to survive. Most commercial seed producers are very careful not to use seeds from plants having a bacterial disease. The use of untreated seeds is becoming more popular as the organic trend continues to grow in popularity. Organic growers often require untreated or organically grown seed. These seeds could be more likely to carry a seed-borne bacterial or fungal disease.

Virus Diseases

A virus disease in the greenhouse can be the most damaging type of plant disease. Once it gets into a plant, a virus continues to grow and spread within the plant as it grows. Cuttings or any vegetative starts taken from a virus-infected plant will be infected. In some instances, tissue culture can be used to produce virus-free plants from virus-infected plants, but details on that are for a more in-depth discussion on tissue culture.

Virus diseases are spread in one of two primary ways. Some viruses, like Tobacco Mosaic Virus, are spread by contact. By working with or handling infected plant material, the virus can get onto your hands or tools and be spread to uninfected plants in a different location. Some viruses will remain viable on uninfected surfaces for hours, days or even months before being transmitted to other plants. You can spread Tobacco Mosaic Virus to plants in your greenhouse if you have handled infected tobacco, potatoes, peppers or eggplant. Wash your hands with Lava brand soap after handling any of the plant parts of the above plants and before handling tomato or petunia plants in your greenhouse to minimize the chances of transmitting the virus.

Once a virus that can be spread by contact has been introduced to the greenhouse, it can then be spread to susceptible plants in the greenhouse through your cultural work and handling of the plants. This often occurs before the first symptoms of the virus are visible and identified in the greenhouse.

Insect vectored viruses are the other type of virus that can be introduced to the greenhouse. This type of virus is spread through the feeding activity of insects like thrips or aphids. Each virus has its vector insect. Often, by the time the virus disease is noticed in the greenhouse, it has spread to several or all susceptible plants. Fortunately, these types of viruses are not widespread, common problems. If you are in an area where one or more of these viruses can be a problem, the best thing to do is to keep the insects out of the greenhouse as was discussed early on in this article.

Using Beneficials in the Greenhouse

Many of the common insect and mite problems in the greenhouse can be controlled with the use of general or specific beneficials. Beneficials can be defined as insects or mites that live on the insects or mites found on the plants in the greenhouse. Many commercial growers use beneficials. A large part of the cost of using beneficials, even for the commercial grower who buys in large quantities, is the shipping. A small grower may need fewer beneficials, but the shipping costs are going to be the same.

There are two overall types of beneficials based on how they are used. There are general purpose beneficials like lacewings. Although lacewings prefer aphids, they will find something else to live on if no aphids are present. They will survive and be ready to welcome any aphids that might find their way into the greenhouse. Beneficials like this can be introduced on a somewhat preventative basis, but may need to be introduced on an ongoing basis because the numbers will not be maintained when their favored food source is not present.

Other beneficials are very specific in what they will control. Some of the small wasps that are used as beneficials paricitize the pest and spend part of their life cycle within the pest. The pest is killed in the process. The beneficials that operate in this manner are very host-specific. If their host is not present in the greenhouse, the beneficial will die without reproducing. Use of such beneficials in a small greenhouse on a preventative basis is not considered a good approach from an economic standpoint.

The hobby greenhouse can bring many hours of enjoyment and much in the way of produce, but this does not come without some challenges. With adjustment in your approach and understanding the needs of your greenhouse during seasonal changes, the challenges will be manageable, and most problems avoidable.

A little knowledge about what to do – and not do – will arm you for success and bring many years of happy growing!