Garden Myths

New and experienced gardeners alike may be tempted to experiment with novel or creative plant care techniques touted online as effective home remedies for a wide range of plant problems. Unfortunately, many of these remedies do not work and even have the potential to do more harm than good. This article addresses some misconceptions commonly encountered when searching for plant remedies online.

Misting houseplants increases relative humidity.

Many common houseplant species come from tropical regions of the earth with high temperatures and humidity. Generally, the temperatures inside our homes are suitable for these plants but relative humidity levels are much lower. This makes for an environment where leaves transpire rapidly and can desiccate quickly because warm, dry air has a high potential for stealing moisture and holding on to it. The simple solution would seem to be to raise the relative humidity of your home for the plant.

Relative humidity commonly found in the home environment is 10-20%. Compare this to a greenhouse which has relative humidity levels above 50%, usually much higher. You would not want these humidity levels in your home as it would encourage mold and algal growth. So, why not apply water directly to plant leaves from a mister to replicate these conditions on a small scale?

Because warm, dry air readily collects moisture and moves around the house, any relative humidity gained in a small area is lost quickly to the surrounding air. Think about a hot shower in a small bathroom and how the steam quickly leaves once the door opens. Soon, temperature and humidity levels return to those of the rest of the house. You would need to apply a constant mist, in a compact, closed area to achieve high relative humidity for house plants. Even plants in the bathroom, from the scenario above, would lose the gained humidity a few minutes after the shower has ended. The temporary fluctuations in humidity from misting do little for the health of the plant long-term.

Misting may even cause some problems for plants. Large water droplets that collect and sit on leaves for long periods of time can cause problems with rot and mold. Remember, liquid water is not the same as water vapor. However, orchids and air plants (Tillandsia) do benefit from misting. Orchids have fleshy roots with a special adaptation called velum which can gain moisture for the plant from misting. Similarly, Tillandsia have trichomes that are adapted to gather moisture from the air. Misting these plants is one way to 'water' them but it is different than raising relative humidity needed by other tropical plants.  For more information see Growing Indoor Plants with Success.

Dish soap is a cheap, safe pesticide.

There is no denying that dish soap can kill pests but it will also kill beneficial insects and quite possibly the plant itself. If a product is not labeled for garden use, it may not be used as a pesticide under law. There are insecticidal soaps with different chemistries labeled for use on garden pests and, when applied to label recommendations, are very safe. Even though insecticidal soaps are cheap, the rationale of using a soap you already have under sink is certainly tempting. But, would you use insecticidal soap on your pots and pans if you were out of dish soap?

Dish soap, bath soap, and insecticidal soap all have different chemistries intended for different uses. Dish soap is actually a strong surfactant, designed to remove tough fats and oils from cookware. Plants have delicate oils and waxy cuticles that are easily stripped by dish soap leaving the plant vulnerable to drying out and sunburn. Dish soaps can also have antimicrobial properties that can interrupt symbiotic relationships that plants may be forming with beneficial microbes.

Many modern bath soaps are saponified using sodium hydroxide, while insecticidal soaps use potassium hydroxide. Both are effective against scale insects but residual sodium ions from the use of bath soaps can harm plants. Insecticidal soaps can be found at your local garden center. They are an inexpensive, safe, and effective means to control insects and mites on plants. Check out Managing Plant Pests with Soaps.

Copper pennies can control tomato blight.

Blight may refer to two different diseases in tomato, the fungus early blight (Alternaria tomatophila and A. solani) or late blight caused by the fungus-like oomycete Phytophthora infestans. Both diseases can be found in Pennsylvania. There are copper-based fungicides registered for use in managing early and late blight. So, the rationale of inserting a copper penny into a cut on the stem of a tomato plant to prevent blight has at least some basis. However, copper fungicide sprays are topical treatments used to prevent blights from establishing on plants. Sprays lose residual effect and must be reapplied to stay effective. Copper is not a systemic fungicide and will not translocate throughout the plant. Copper from pennies will also not translocate throughout the plant. What's more, copper sprays are formulated as copper hydroxide or copper oxychloride to make them more effective when applied. Placing a chunk of copper in the plant or copper mesh around the plant is not an effective application. Additionally, making unnecessary wounds to your plants can create entrance ways for other diseases or pests.

If early or late blight is a problem in your garden, practice crop rotation, proper culture, and blight resistant tomato varieties. Early blight is easily contracted when plant leaves are in contact with the ground. Keep plants staked and lower leaves trimmed as needed. Keep foliage dry when watering. Late blight overwinters easily in infected potato tubers. Under the right conditions late blight can take over a plant quickly and you may need to destroy infected plants. Do not grow solanaceous vegetables such as tomatoes, potatoes, peppers, and eggplant in the same patch of garden within 2 years of the last solanaceous crop. Finally, consider planting blight resistant tomato varieties to curb the spread of the disease in your garden.

For more garden myths see Debunking Garden Myths.