Mulches are another option. According to Dr. Andrew Senesac of the Long Island Research Laboratory, black plastic, remay spun and glued, and Weedblock can be utilized successfully in controlling weeds around field grown herbaceous perennials. In some studies however, Achillea and Stachys flower production was reduced by these mulches due to the physical restriction of the spreading shoots. Spreading perennials that do not produce adventitious roots did not show any significant yield differences from the controls. In addition to the previously-mentioned materials, there are a number of other mulch materials that can be utilized with varying degrees of success.
Some growers use rotted sawdust, bark mulches, wood chips, spoiled hay and straw.
If not composted properly, sawdust will rob the soil of nitrogen. Bark mulches and wood chips can be used but their cost prohibits usage by most growers. Clean, weed-free straw is the most effective mulch material. Hay generally contains too many weed seeds and often increases the weed pressure. Organic mulches can be applied to weed-free, warm soil soon after planting.