Cover Crops in the Vineyards

In his summer editions of the Douro Insider reports, Miles Edlmann occasionally alludes to the need to “mow the lawn” in Graham’s vineyards.  This may sound odd, but it is in fact one part of a very important and innovative system we have developed of using cover crops to manage the health of our vineyard environments.

There are several facets to the programme:  nutrition, disease and pest control, and water management, both conserving water in the dry summer months and managing drainage when we have heavy rains, usually in the winter.  Miles’s research into the effective selection and use of cover crops as a single solution to all these issues has made a significant difference in the quality of our vineyards.  This investment in research and the strategic application of our findings is one more factor which contributes to the outstanding quality of Graham’s Ports.

Nourishing the Vineyards

Look carefully, you can see the denser, darker clover at the far end of the row, and the change to grasses about half way down these vertical plantings at Vila Velha

The cover crops provide nutrients and improve soil quality for the vines in several ways over their lifecycle.  In a newly planted vineyard we use clover, which fixes nitrogen in the soil, to help the new vines get a healthy start.  When the crops are cut, the fallen plants form a natural mulch which ultimately decomposes and increases organic matter in the soil.

In mature vineyards we may mix clovers with grasses such as oats or ryegrass to either encourage or control the vigour of the nearby vines.  In the vertical vineyards particularly, where organic matter and nutrients tend to wash down the hillside, we plant clover at the top to nourish the vines in the poorer soil, and gradually introduce more vigorous grasses to the crop mix near the foot of the hill to compete with and thereby control the vigour of the vines at the nutrient-rich foot of the hill.

Cover crops also provide a little healthy competition for the vine roots in the upper soil, forcing young vines to reach even deeper than the grasses to find an adequate water supply, which is crucial for the vineyards’ long-term survival.

If you were a bee, wouldn't this be your idea of heaven?

Finally, the bees love the crops, particularly the clovers, and their presence in the vineyards is always desirable – not least because their hives give us the wonderful honey served at meals in our quintas!

Disease and Pest Control

Cover crops work in several ways to control both disease and pests in the vineyards.  First, by providing a distraction from the vines for potential pests:  in a chemically-managed vineyard weedkillers would be sprayed to prevent anything growing between or under the vines, particularly in the spring, during budburst.  This effectively left no food for hungry pests like the vine weevil except the young buds of the grapevines.  By planting cover crops we give the little darlings something else to eat, and lots of it, so they won’t go for the vines; also the crops provide a good habitat for the natural predators of the undesirable pests.

By sowing our own chosen cover crops, we can crowd out the growth of more pernicious weeds that would cause us greater trouble in the vineyards.  In this way we can minimise the use of herbicides, as our only other means of dealing with the really undesirable species is very time consuming manual weeding or cutting down.

Some viticulteurs argue to keep both under and between vine spaces clear of growth, lest the vegetation create a too-humid atmosphere that might encourage either oidio (powdery mildew) or mildio (downy mildew).  In general, we find this isn’t a concern – we do keep the under-vine space clear, and plant crops only between rows, which will be mown for the first time before they reach bunch or leaf-level of the vines.  Furthermore, the summer season – and the vegetation itself – is usually much too dry for this to be an issue in the Douro.

Water Management

In heavy December rains Miles admires the way the cover crops control the water flow in these vertical plantings

The cover crops are an incredible asset in aiding both drainage and water conservation.

When it rains, we want the water to soak into the soil rather than run off down a hillside.  The plants help accomplish this in several ways.  First, rain which falls and is caught and held in the grasses will course down the stalks of the plants and ultimately into the soil, where we want it.  Second, in heavy rains, the plantings form an obstacle course for any running water and slow it down, which gives it more time to soak in, and also minimises the erosion of topsoil and loss of organic matter in runoff from our vineyards, particularly any vertical plantatations.  Finally, the plant material  which was cut and left lying between rows as mulch acts as a sponge, soaking up water during the rains, and releasing it slowly afterwards, into the soil.

During the incredibly hot dry summers the cover crops, even when cut down and left between the vines, shade and hold humidity in the soil.  Our research has proven significantly lower soil temperatures and higher residual humidity where we have cover crops, which will help the vines cope with the summer drought conditions.  In addition the stubble impedes air movement at soil level, so the hot dry winds typical of mid-summer Douro afternoons cannot dry out the soil as rapidly as they do on barren hillsides.

All that in a blade of grass!