PHOTO: kieferpix/iStock/Thinkstock
Dawn Combs
July 16, 2014

I first worked in a greenhouse in college. I had always been attracted to these warm sunny spaces and all the curious plants that could be sustained regardless of their native climate. I knew my duties would include daily watering and, at times, propagating and planting new species. What I didn’t expect was that most of my time would be spent fighting the greenhouse’s pest and fungus problems.

Of course, now it only seems logical. A greenhouse is only mimicking a natural environment. It can be really great, don’t get me wrong, but greenhouses don’t allow for too many natural breezes, or let the rain clean the leaves and drench the soil, or maintain a natural pest-to-predator balance.

In that college greenhouse, I often removed unwanted bugs manually, but we also used a fair amount of chemicals. This is a practice I didn’t want to implement in my in my own greenhouse, so I accepted the challenge of finding natural pest and disease solutions.

Greenhouse pest and disease problems will vary depending on how the greenhouse is used, so you’ll need to experiment with the methods that work best for you. Keeping plants in one place allows pest insects to get a foothold, so if possible you’ll want to rotate where you plant. Another important crop-health trick in any environment is to employ soil conditioners (compostfish emulsion or composted manure) to keep the soil healthy. Also bear in mind that any pest-control measure aimed at killing or repelling unwanted insects can also affect beneficial insects you’d like to attract. Be sure that your problem warrants these types of solutions, natural or otherwise, before applying.

Here are six tricks I’ve used to fight disease and pests naturally in my greenhouse.

1. Chamomile Tea

If you’re planting new seeds or starting cuttings, damping off can be a major threat. This fungal growth is fostered by the greenhouse’s warm, moist environment. My favorite natural defense against the fungus that causes damping off is to water seedlings on a regular rotation with a simple chamomile (Matricaria recutita) tea. Chamomile tea is an anti-fungal shown to be particularly effective with young seedlings. To make the tea, pour boiling water over 2 to 3 teaspoons of dried chamomile, cover and steep for 10 to 15 minutes.

2. Horsetail or Nettle Tea

If you see evidence of mildew on the leaves or fungus at the soil level of any mature plants, you can use one of two teas to spray on the leaves or the soil, wherever fungus is a problem. I prefer horsetail (Equisetum arvense) because it’s an anti-fungal that has been shown to be highly effective over many years of use by biodynamic farmers. However, other people have used nettle (Urtica dioica) tea successfully. To prevent mildew or fungus altogether, water your plants early in the day. If you must water in the evening, avoid wetting the leaves and water at soil level only.

3. Rhubarb Tea

The most common insects to cause problems in the greenhouse are aphids, mites and whiteflies. If these pests build up their numbers, it can be a full- time job keeping up with them. By all means, you’ll want to focus on reducing the population, but don’t overlook the problem that brought them to the greenhouse in the first place.

A rhubarb tea will help against aphids, spider mites, june beetles and thrips. Use it only on non-edible plants.

Ingredients

  • 1 cup rhubarb leaves
  • water to cover
  • 1/4 cup castille soap

Preparation

Cover leaves with water in a stock pot with a lid and bring to a boil. Reduce heat and simmer for 20 minutes. Cool completely and add the castille soap. Strain and add to a spray bottle, with a warning label that indicates use for only non-edible plants.

4. Citrus Spray

This easy citrus spray will help against whiteflies.

Ingredients

  • 2 cups citrus peels
  • 4 cups water

Preparation

Boil water, pour over peels, and cover. Steep until cool. Strain and put into a spray bottle.

5. Beneficial Insects

There are several different predatory insects you can release into your greenhouse, each one specializing in a different pest. Ladybugs are one of the most common and they will help with aphids, cabbage loopers, spider mites, scale, thrips and whitefly larvae. It’s said that as long as you have a sufficient supply of pests for them to eat, these beneficial bugs will thrive, though I feel it is important to note that they need more than that.

Most beneficial insects will benefit from a companion plant “island” in the greenhouse, just as they do in the garden. This can mean planting a small area with plants in the parsley family, such as dill or fennel, as well as scented geraniums or marigolds. Strange as it may seem, I usually let some thistles (Cirsium arvense) grow in this space, as well, as this is the plant where I see ladybugs go to hide most often.

6. DIY Whitefly Traps

In the video below, I’ll demonstrate how to make non-toxic whitefly traps that you can hang in your greenhouse.

7. Quarantine Plants

Plants that have been brought in from outside gardens should be quarantined until you know they’re not carrying pests. If you have any weak or diseased plants, remove them immediately. Above all, disease and pests will not attack plants that are fully healthy. Always focus on your soil health. Fixing any nutrient deficiencies will reward you with a reduction in bug problems.



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