Nematodes are tiny worms that establish themselves in the planting soil around a wide range of plants. They are extremely difficult to get rid of once established and can be spread easily by irrigation run-off, infected tools and the introduction of infected plants. Of the many types of nematodes, a few prey particularly on the roots of roses, preventing them from absorbing water and nutrients. This harms the appearance and performance of the roses and can eventually kill them. The only thorough means of eradication is professional soil fumigation before planting and then planting roses known to be nematode-free.
Fumigate the infected soil with a product containing Vapam. Soil fumigation products are not sold in retail garden centers and must be administered by a trained professional.
Lay down dark plastic tarping in the late spring or early summer, leaving it in place for four to six weeks during peak heat. Rose nematodes and their eggs will die when exposed to soil that gets hotter than 125 degrees Fahrenheit for 30 minutes or at 130 degrees Fahrenheit for five minutes. The longer the tarp is left on the better, but remove it in the fall to allow the soil to breathe.
Plant French marigolds as companions plantings or as a fallow crop to prepare the soil for future rose plantings. Look for cultivars such as Nemagold, Queen Sophia or Tangerine. Deadhead some or all of the marigold flowers at bloom to prevent broad self-sowing or not, as desired. Marigolds can suppress nematodes.
Refrain from moving and transplanting infected roses to new soil as they will simply spread the nematode infestation to the new soil exacerbating the problem. Destroy infected rose plants, solar-treat or fumigate the soil and start with rose plants known to be free of nematodes.