Draining or Retaining? Fungus Gnats and Other Side Effects of Poor Drainage

Southwest Yard & Garden by Dr. Marisa Thompson

"The drainage rate of any soil is also influenced by the drainage rate of the soil lower in the profile... To understand soil drainage one must investigate the total profile." - the Internation Society of Arboriculture and Urban Tree Foundation "Planting Standards" 

Even African violets, which require relatively moist soil, need good drainage and to dry slightly between waterings in order to thrive (photo credit M. Thompson).

This split-leaf philodendron thrives when soil is allowed to really dry out between waterings (M. Thompson). 

Question:

I’ve been battling with little gnats in potted plants for months. I have covered the soil with small aquarium gravel, and have tried using bowls of vinegar and even sticky flytraps. But they are still very prevalent in my home and making me a little bit nuts!

-          Erin F., Albuquerque, NM

Answer:

It sounds like you have fungus gnats. As Dr. Carol Sutherland, NMSU Extension Entomologist and State Entomologist for NMDA, explained, “Fungus gnats are those very tiny, blackish, mosquito-like creatures that fly in your face when you water your plants indoors. Female fungus gnats are attracted to the organic matter in your potting medium. Algae, fungi, and various microbial growths are potential food sources for the adults, while the moist organic matter in the medium is an attractive place to lay their eggs. Their minute, thread-like larvae feed mainly on this organic matter. It doesn’t take long for fungus gnats to find houseplant collections and become annoying pests. At least fungus gnats do not bite or sting.”

Sticky traps or other trapping methods only reduce the adult populations; they don’t control the eggs in the soil. There are pesticide options, but even if effective at controlling gnats in the short term, such pesticides do not address the underlying issue, so you’re likely to get the problem again and again.

The underlying cause is constant moist conditions down in the soil that encourages fungal growth. Today I’ll concentrate on the three most common mistakes when it comes to moisture buildup in potted plants and consequential outbreaks of fungus gnats.

1)      Overwatering. Of course, plant roots need water. However, roots also need oxygen. Potting soil should dry out slightly (or completely, depending on the plant species) between waterings so that roots get enough exposure to oxygen. Imagine you have an empty bucket and you throw in a few shovelfuls of soil. If you added water to fill the bucket and let it sit, what will happen? Eventually, the air between the soil particles will bubble up, and over time the soil will pancake down at the bottom. The soil at the bottom of the bucket has become compacted. We know compacted soil is bad for plant roots in the landscape, and the same is true for potted plants.

How do you know if you’re overwatering? Lifting the pot to check its lightness can help you get a sense for how much moisture is still in the root zone (although not so easy with terracotta or extra-big pots). Some people use inexpensive water meters available at most plant nurseries, while others find them unreliable. Sticking a finger down in the soil to check moisture content is also a great indicator tool.

2)      No holes in the bottom of the pot. Overwatering is even harder to control without adequate drainage holes allowing for proper root aeration. Like I mentioned before, roots need oxygen (with few exceptions). I’m not sure why planting containers are sometimes sold without holes, but holes are crucial for survival. If you don’t see water coming out into the saucer after watering, first check to be sure that there are actually holes in the bottom of the pot and then that the drainage holes aren’t blocked.

3)      Gravel at the bottom of the container “for drainage.” Gravel or other chunky material at the bottom of potted plants does not facilitate drainage. Shocked? So was I. More often than not, plant guides recommend this practice for improved drainage, but it just plain ain’t true.

The reality is that while water follows the basic rule of gravity, it also follows rules of adhesion and cohesion. In the essay entitled “The Myth of Drainage Material in Container Plantings,” Dr. Linda Chalker-Scott states, “water does not move easily from layers of finer textured materials to layers of more coarse texture.” Where soils of different textures meet, there is a soil interface that actually inhibits water movement. This is true for houseplants and true in the landscape too! (see figure below). Gravity can pull excess water down into the gravel, but some water will stay “stuck” in the finer zone. Therefore, in a gravel-lined container, at the bottom of the soil layer moisture is caught there and stagnates, which creates a great growing condition for fungus and the gnats that love it.

"A layer of silt loam sits above a layer of sand, and water from an Erlenmeyer flask drips in. Intuition says that when the water reaches the sand, it will move more quickly through the sand because the pore spaces are larger than those in the silt loam. But intuition is wrong, as this series of photographs clearly demonstrate. Water is finally forced into the sand layer by gravitational pressure, after, of course, saturating the silt loam" (Images and quote from http://gardenprofessors.com/container-planting-intuition-vs-reality/). 

As I’m typing this, I get the feeling that the dwarf banana tree in my office window is glaring at me. I repotted that sweetie way back several years ago, back when I too used gravel as a container liner, so I bet it has gravel in there. After 11 years of varied neglect and occasional overwatering, the gravel obviously didn’t kill the plant. Now I know that the gravel certainly didn’t help with drainage. And, come to think of it, I have battled fungus gnat infestations on more than one occasion.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the NM Desert Blooms Facebook page (@NMDesertBlooms)

Please copy your County Extension Agent (http://aces.nmsu.edu/county/) and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505- 865-7340, ext. 113.