Digging In - Protecting Turnips From Root Maggot Flies

By Scott Aker
Special to The Washington Post
Thursday, December 4, 2008

Q My 98-year-old father has been gardening all his life but is having trouble controlling insects now that many of the insecticides that he used before are no longer available. He is particularly concerned about his turnips. As the turnips mature in the fall, something is eating them and tunneling through them. He has to cut away half of the turnip to eat them. In some cases, the turnips are completely hollowed out.

A The pest in question is the root maggot fly. Cover the seedlings with a floating row cover to keep the flies from reaching the soil surface, where they lay eggs near the young plants. The row cover will also keep out flea beetles, the other major pest of turnips.

The fly is the same pest that tunnels in the roots of the cole crops. In the case of cabbages and related plants, a standard treatment is to place tar paper around each plant to keep flies from reaching the soil surface. Since turnips are typically sown and then thinned, the tar paper trick is not really feasible for them.

Temperatures higher than 95 degrees will kill the eggs of root maggots. In September, cover the row with clear plastic for a few days after the seedbed is prepared but before seeds are planted to kill eggs that might be present. Then use the floating row cover as the turnips develop.

I had a large sweet gum tree removed and the stump ground a few inches below the soil line. But much of the stump and its large roots remain. How long will it take to rot so I can landscape the area?

You can speed the decay of your stump by using a chain saw to make cuts on top of the stump about two inches deep, spaced two or three inches apart. Carve a second set of cuts at right angles to the first set.

If you aren't comfortable with a chain saw, you can use a portable drill and the largest bit you have and drill many holes into the stump. The next step is to add nitrogen, which will further speed rot. Commercial stump rotting compounds are available; they generally consist of little more than potassium nitrate, more commonly known as saltpeter. But most fertilizers also contain nitrates. Pour the grains into the cuts or holes. Every time it rains or water is applied to the stump, some of the nitrates will soak into the wood, speeding decay.

Your stump was ground down, and that will further speed decay because the remnants of the tree are almost entirely underground, away from sunlight that has a tendency to kill the microorganisms that break down wood. You may still want to apply fertilizer to the area to speed the process.

Trees with light-colored wood such as pines, maples and birches decay more quickly than oaks, cedars and black locust. The latter all have lignins, tannins and phenolic compounds in their wood that prevent decay. Sweet gum is in the middle of the road in terms of its speed of decay, and you can expect it to take five or six years to rot with ample nitrogen and water.

In the meantime, there is no harm in planting trees and shrubs among the dying roots. Apply extra fertilizer until the roots rot away because decay organisms compete with the plants for nutrients.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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