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When vigor is very low : Grass cover can be risky as it can cause a decrease in production and plant reserves. The soil's water reserves are all the more affected by the presence of a cover crop when the weather conditions are challenging. For example, in a hot vintage, grass cover can stop vines with poor vigor from ripening correctly. Other solutions can be envisaged, in particular mulching (see section on alternatives to grass cover).
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If water and nitrogen supplies are limited as well as shallow rooting : If a plot demonstrates poor agronomic qualities (low mineral reserves, low CEC, slow mineralization, etc.) but a grass cover crop is desired, care should be taken to choose one that does not overly compete with the vine. This also applies in the case of shallow soils, very sandy soils and/or when the plot is situated at the top of the slope.
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If there is a risk of late frosts : Cover crops maintain ambiant humidity. When they transpire this humidity evaporates, which results in a temperature drop. This can cause frosts in spring when the temperature is close to 0°C. There is a greater risk when grass cover is high and close to the buds. Lower vines are therefore more likely to be affected by frost. This does not mean that grass cover is unsuitable, but, that it should be kept low to the ground to limit this risk (mowing, working the soil by hilling/de-hilling). A trailing grass cover can mitigate this risk. Broadly-speaking, a distance of 30 cm between the buds and the plant cover is recommended. Also, hedges and shrubs or shelterbelts can obstruct the flow of cold air, thereby increasing the risk of frost.
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