Heat and Humidity

Heat and Humidity

Heat and Humidity

Everybody talks about the weather. People talk about weather because it’s safe if you need filler for a lagging conversation. This is especially true when it gets hot and humid. Humidity is the relative amount of water vapor contained in the air. As temperature rises the capacity of the air to hold water vapor increases.  Measured on a scale of relative humidity, it ranges from 0% which is dry, to 100% that is completely saturated. So weather talk is ubiquitous because, well, weather affects everyone, your mood, your comfort level and even your conversations because it’s a neutral topic. It also affects your plants. 

With plants the thinner the leaves are, the greater the likelihood of it needing higher humidity. Plants with thicker, waxier leaves are better able to withstand dry air! The sword fern Nephrolepis exaltata are known as “Boston” because a pioneering Florida nurseryman years ago shared them with a friend in Cambridge Massachusetts where they were propagated by a local distributor. The ferns are notorious for wanting high levels of humidity and if they don’t get it they have a bad hair day excessively dropping leaves and making a mess. They like hanging out on the front porch but are generally not used in interior landscaping for that reason. Indoors with our houseplants summer air conditioning and winter heating is not conducive to a humid environment. Ficus, Palms, Anthurium or Philodendron to name a few are tropical region plants that benefit from high humidity because it reduces transpiration or water loss, browning of the edges of foliage and can discourage spider mites who love it dry. They let you know when they’re not happy. Some plants are acclimated to dry arid air with little humidity. Generally these plants like succulents have thick waxy leaves and other adaptations for water retention, not the paper thin leaves of a fern.

You of course can help because the main defense plants have against high temperatures is water. They cool themselves by allowing water to evaporate from their leaves. As the temperature rises, water evaporates faster requiring more water to quench their thirst. Make sure there is enough water in the soil to run a plant’s “air conditioning” system. Transpiration of water through the leaves cools a plant and the vacuum created as vapor escapes pulls water up through the plant tissues, distributing nutrients. Water pressure helps them stand up to the heat and not wilt.

Look at the shape of leaves too. I have always noticed many tropical plants (which we refer to here in the north as “houseplants”) have pointed leaves or heart shaped leaves.  That’s because plants that are native to areas of high humidity have leaves with pointed ends we like to call ‘drip tips’. It’s like sweat dripping off your nose on a hot and humid day. Drip tips naturally allow excess water to flow off the leaf, preventing excess moisture sitting on the leaf. Plants with cupped foliage or soft hairy foliage and on “drip tip” can be more prone to fungal diseases. Think cucumber or melon foliage in your vegetable garden. Succulents and Cactus like dry conditions so if overly wet or overwatered disease and rot or fungus can quickly become an issue. Plants native to tropical rain forests can easily deal with stifling heat and humidity. Some of our  temperate-zone plants can become scorched or wilted if we don’t keep an eye on them in extreme temperatures.

Here are some thoughts on plants dealing with heat and humidity:

Plants struggling in the heat will lose their green color and have a silvery cast or pale color.

Don’t forget that plants next to a bright reflective wall or concrete driveway that absorbs and radiates heat may be hit with temperatures that are greater than the reading on your thermometer and may need some extra attention.

Leaves may curl or they may even fold on their central vein to conserve moisture in excessive heat and humidity. When they show scorched or brown areas the damage has already been done. Work to rescue the plant from total collapse.

Mulch helps cool the soil temperature and retain needed moisture around the roots.

Flowers are good indicators or “triggers” to necessary action when they start to dry up on a plant or flowering ceases.

Ironically wilting also can be an indication of overwatering if the soil has poor drainage and that’s why paying attention to good soil structure whenever planting or potting is important providing a good foundation to endure the occasional stressful day.