Ask Skip: Styrofoam and Root-Knot Nematodes

By Skip Richter, Contributing Editor

Q: I’ve read that you should put Styrofoam peanuts or gravel in the bottom of a pot for drainage. Is that true?

A: Water moves out of soil when it is saturated, and gravity pulls it away from the soil particles it is adhering to. Water doesn’t move from fine-textured potting soil into underlying gravel or Styrofoam until it literally drips out. So, adding gravel doesn’t facilitate drainage, but rather results in less potting medium to hold water and nutrients for plant roots to utilize. Doing something to lessen the soil volume isn’t helpful, so I encourage folks to use larger containers in our hot climate. If you are concerned about potting soil washing out of a drainage hole, place a section of window screen in the bottom of the pot.

Q: Is there a product that will kill root-knot nematodes? If not, what is the best way to get rid of nematodes?

A: There is not an effective home-garden chemical control for nematodes. Root-knot nematodes can be reduced somewhat through various practices such as:

• Soil solarization with clear plastic for 4–6 weeks in summer, rototilling every two weeks in summer to expose them to the hot, desiccating sun.
• Planting mustard and rototilling it into the soil to release chemicals that are detrimental to nematodes
• Planting resistant species and varieties of plants for several years to avoid building up nematode numbers.
• Incorporating lots of compost and/or organic matter into the soil to promote the increase of certain antagonistic organisms in the soil.
• Planting “trap crops,” including some varieties of French marigolds (Tagetes patula) (such as ‘Tangerine,’ ‘Lemon Drop,’ ‘Single Gold,’ ‘Petite French’) in summer or cereal rye (‘Elbon’ is one variety) in the cool season. While French marigolds are generally better, some types of African marigold (Tagetes erecta), such as ‘Crackerjack,’ can also reduce nematode numbers.

Note that it is ineffective to just plant some marigolds alongside your susceptible plants. You need to plant a solid cover of marigolds (6-inches or less apart) throughout the bed to fill the soil with marigold roots and you need to grow them for at least three months.

The above practices suppress or reduce nematode numbers but do not eradicate them. Nematodes can be quite prolific and populations will quickly return when a susceptible crop or weed species is grown in the area.