by Andrea Laine, EMGV
In May I received a quick reminder of the downsides of overwatering. Temperatures soaring to 15 degrees above the seasonal average in my western North Carolina neighborhood caused me to give my container garden a second watering in less than a week since the last watering. Now, that might be called for in the middle of a hot summer given that plants growing in containers do not have access to a reservoir of groundwater. But it was still, technically, spring and, more importantly, my plants showed no signs of suffering. I watered in the morning as recommended and by afternoon I had mushrooms popping up in the plastic containers. I plucked them out; Two days later, there were even more!
On closer inspection I noticed something else – a number of teensy flies flitting all around the soil surface and sides of the containers. By overwatering, I created a perfect setting for fungus gnats.
Fungus gnats thrive in very moist, warm conditions and feed on fungi. Mushrooms are fungi. So weeding out the mushrooms was a good move; But not good enough. These gnats like to stay put. They live and rapidly breed in the top inch of soil and feed on organic matter and sometimes roots, too. A generation of fungus gnats (from female to female) can be produced in about 17 days depending upon temperature.1 In the right conditions, three to four generations can be produced in a year. As many as 272 eggs have been counted in a single female fly 2.
Another way I may have unwittingly invited the gnats to my garden is by neglecting two pots that contained perennials that did not successfully overwinter. The rotting roots of those plants could have been gnat food. I should have emptied those pots out weeks earlier. (I was hopeful that the plants might miraculously recover.)
Fungus gnats are most often associated with houseplants where air circulation may not be ideal. My container garden was on a covered deck, so it mimics some of those conditions.
In the long run, the fix for the problem I had created was relatively easy: 1) I continued plucking mushrooms from the containers as they appeared. 2) I applied a neem oil drench to the soil every seven days for one month. Neem oil is a naturally occurring pesticide found in seeds from the neem tree3. 3) When temperatures returned to normal, I moved all the plants off the covered deck and into the yard where they would receive bright sunshine for most of the day and better air circulation. 4) I restrained myself from overwatering!
Less is more: Water amply yet less frequently. Be sure your containers have holes in the bottom where excess water can drain. Be especially careful about watering plants potted in plastic containers. Plastic is not porous and will retain moisture. It will take longer for plants in plastic pots to dry out.
Several plants were effected by my error, most were herbs and all have recovered. Only the borage (Borago officinalis) got noticeably worse before it got better. Its form is a little wacky now, but as of this writing (July 7) it is blooming beautifully and is visited by many bees.
Footnotes and Additional Resources
Gnats on houseplants: