Dew or Dew Not: Why to Avoid Wetting Foliage

In an attempt to mimic the beneficial effects of rain and dew — and probably also influenced by the true benefits of foliar feeding — some gardeners have begun wetting the foliage of their plants, often to the point of soaking them and almost always to their detriment.

Unless you are wiping off dirt and debris with a damp cloth or using a fine mist sprayer to give your indoor ferns, bromeliads or orchids an occasional and temporary humidity boost (ferns thrive in shady, moist climates, like the rainforest or understories of forests; bromeliads and orchids have adapted to capture and use water on their leaves), we implore you to stop. Keeping foliage wet can generally do more harm than good.

Nature’s fungal incubator

Obviously, water is essential to the growth and survival of healthy plants. We’d never advocate against a watering schedule appropriate for your plants’ requirements and your environment.

There is, however, a difference between a short-lived wetting and prolonged moisture.

What we’re concerned about is frequent, heavy misting of leaves, pouring water on leaves, or keeping leaves wet for extended periods. Each can promote fungal disease, reduce the rate of photosynthesis, and lead to leaf scorch, and none is effective at getting enough moisture to the roots.  

Fungal disease

Wet leaves create conditions just right for the development of fungal disease. Not only are wet leaves nature’s incubator for fungal spores — the reproductive units that prefer moist conditions for germinating and spreading fungal disease — but a continuous film of water on the leaves of terrestrial plants can break down their waxy coating, which is their first line of defense against pathogens. 

Together, this makes the plant more susceptible to fungal disease. And the risk is even higher for plants with fuzzy leaves whose hair traps water droplets on the leaf surface for longer periods. The more they stay wet, the greater the likelihood that fungal spores will germinate and spread disease

Reduced rate of photosynthesis

Photosynthesis is a complex process, and how much wet foliage affects it varies by plant type, light exposure, and other factors.

However, when leaves are wet, their stomata — the tiny pores that allow plants to take in carbon dioxide (CO2) for photosynthesis and release oxygen (O2) as a byproduct — partially close. (It may seem counterintuitive, but they do it prevent excessive water loss.) This restricts the intake of CO2, slowing the rate of photosynthesis. 

Leaf scorch

Finally, water droplets on the leaf surface act like tiny magnifying lenses. In bright sunlight, these lenses intensify the light and can potentially scorch the leaf cells. To protect themselves from this damage, plants can close their stomata even more, which reduces CO2 intake and photosynthesis. 

Humidity without harm

Clearly, then, directly watering foliage is not recommended for routine plant care. But there are options if you are concerned your plants aren’t getting enough humidity:

• Group your plants together. As they transpire (that is, release water vapor), it will create a microclimate with higher humidity.
• Fill a tray with pebbles and water. Place your potted plants on top of the pebbles. As the water evaporates, it will increase the humidity around the plants. 
• Relocate your plants. Consider moving your plants to a naturally more humid location in your home, such as a bathroom. 
• Create a terrarium. This is a great solution for small plants that thrive in high humidity. A closed terrarium creates a self-sustaining miniature environment with high humidity levels.
• Invest in a humidifier. Position a cool mist humidifier near your plants but make sure the mist doesn’t reach them.

If you remain unconvinced, mist if you must. But do it with extreme care: 

• Use a suitable spray bottle. Garden shops carry fine mist sprayers that won’t damage delicate leaves. Don’t use a common spray bottle and stay far away from anything with a jet setting. 
• Remember, a light touch is best. Mist the tops and undersides of the leaves only until they are lightly damp. If they’re dripping, you’ve overdone it.
• Time it right. If you’re misting outdoor plants, only do it in the morning. This allows the foliage to dry before nightfall, which can help reduce the risk of fungal disease. 

Best of all, leave the dew to do what it does best, and water the soil directly at the base of your plants. This allows water to reach the roots, where it's absorbed for plant growth.