Humidity And Houseplants, Growing Joy Podcast with Leslie Halleck

January 30, 2024

Why Humidity Matters for Houseplants

Check out my recent appearance on Episode 222 of the Growing Joy Podcast

Here is a short synopsis from Maria of my latest appearance on the Growing Joy Podcast, to talk about humidity for houseplants:

Do I really need a humidifier? Why do my plants get crispy leaves when I crank the heat? When it comes to houseplant care, humidity is right up there with water and light as one of the most important factors. I’ve spent years stressing about humidity levels. Fortunately, I invited my dear plant friend and humidity expert, Leslie Halleck, for another session of Grow Better. Understanding the science behind humidity has helped me stop freaking out and keep my plant collection happy.

Many popular houseplants are tropicals that originate in humid, rainy environments. Places like the steamy rainforest floor where humidity levels hit 90%! No wonder these plants rebel when we try to grow them in our homes.

Humidity affects the process called transpiration, which is the movement of water from the soil, through the roots, up the stem, and out through pores in the leaves. When humidity is too low, transpiration happens faster than plants would like.

Their leaves lose moisture quickly, which causes wilting, yellowing, and crispiness (I see you, my diva Calathea).

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Understanding Key Humidity Concepts such as Vapor Pressure Deficit

RH: Relative humidity tells you the percentage of moisture in the air based on temperature.

AH: Absolute humidity is the actual volume of water vapor droplets that are in a given amount of air.

VPD: Lastly, vapor pressure deficit (VPD) indicates how far the air is from saturation – like how much moisture is missing. So the higher the VPD, the faster your plant will lose moisture from its leaves.

Example: Warm air can hold more moisture than cooler air. So even if relative humidity reads 40% in a hot room and 40% in a cold room, plants will lose water faster in the hot room because of the higher VPD.o check out her new book, Tiny Plants.

What Doesn't Work to Boost Humidity

I was shocked to learn that some popular blogs about boosting humidity just don't work and can be more of a placebo.

Misting plants only increases humidity briefly before settling back down, risks inviting fungal diseases onto leaves, and misses most leaf pores anyway.

Pretty pebble trays are equally lackluster despite their popularity on blogs. Any moisture they add to the surrounding air dries up quickly without noticeably impacting VPD.

Leslie says humidifiers can work but only in large capacity, used regularly, and grouped closely with plants. But even humidifiers struggle to overcome the dry air gushing inside from HVAC systems all day long.

Why Winter Air is Drier

Winter often brings dry air indoors. Natural humidity outdoors is typically lower, and heating furnaces dry the air as it passes through them. Plus the warm air running across leaf surfaces increases transpiration. All that blasting heat from furnaces keeps VPD high indoors, and HVAC systems are built to actively strip moisture out. This explains why our homes tend to be MORE dry in winter than in the summer, even though it's more humid outdoors in the summer.

Managing Moisture Indoors

So what does work to boost moisture for houseplants? Here's what really works:

Growing Under Glass: For humidity-loving plants like aroids and Calathea, Leslie said the best tactic is growing them under glass to contain moisture around the plants.

Creating mini greenhouse environments seals in moisture availability. This could involve a glass cabinet, terrarium, cloche, or Wardian case. I was amazed to learn that these diva plants thrive with less maintenance when grown this way!

The glass recycles humidity at ideal levels to support healthy transpiration. As a bonus, this method prevents messy watering spills around your home.

As Leslie reminded me, getting to know your plants means catering conditions to their preferences. For many indoor plants, high humidity is non-negotiable

PC: Leslie F. Halleck, "Tiny Plants" - Microgramma heterophylla

Light Still Rules

Leslie reminded us that sufficient light still governs everything for houseplant health. No amount of perfect humidity can save a plant that doesn't get enough light! So address both moisture AND adequate sunlight to help plants thrive indoors. I hope you found this episode as helpful as I did! Leslie is truly a humidity expert. If you want to know more, she offers online plant classes via UCLA Extension open to all. You can also book private online plant parties with her on her website to get advice from fellow plant lovers.

Important Links mentioned in the episode

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