The mild winter is causing cover crops to be much more vigorous than last year around this time. The question is - when to pull the trigger to kill those cover crops.

From the soil-quality point of view, it's beneficial to have large quantities of biomass and massive root systems to improve soil structure and add organic matter. The mulch left after killing cover crops is essential to protect our soils from erosion.

So in a way, it's fine to let the cover crops go. On the downside, however, cover crops take up soil moisture when alive and if this reserve is depleted; our main crop might run short. However, the mulch left after killing the cover crop helps conserve soil moisture during the summer and increases infiltration.

Research has shown that increasing mulch cover from 50 to 100% leads to substantial moisture savings. To achieve mulch cover in the summer the cover crop needs to be large and preferably have a high C:N ratio so it does not break down too fast.

All this leads us to recommend not killing the cover crop very early. For example, in Pennsylvania, State Meteorologist Paul Knight forecasts rather high temperatures this spring and below average rainfall (especially in the east of the state) and a hot June, it will be beneficial to have good mulch cover.

So I recommend killing the cover crop three weeks before planting on shallow, droughty soils with low moisture reserves and two weeks before planting on deep soils with large moisture reserves. If you till the cover crop in you will not get any moisture conservation, although its soil improvement benefits are still valid.