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Grounded Grapegrowing GLENN MCGOURT Y Proper Regulated Deficit Irrigation the full potential vine water use. Water def- icits at specific times can be used to posi- tively influence vine growth. Unlike many other fruit crops, the object of winegrow- ing is not simply to maximize yields. Win- eries want to purchase the best possible fruit that they can obtain for a fair price. When using RDI, vine canopies tend to I be more open, and red fruit often colors well. Grape clusters tend to be lighter and looser, which can improve wine quality, as the skin-to-juice ratios are much more fa- vorable than more liberally irrigated fruit. There is also less potential for bunch rot and powdery mildew. By reducing vegeta- tive growth, you may reduce the cost of canopy management including hedging, leaf-pulling and sterile shoot removal. The wine made from this fruit will be consider- ably better than wine made from fruit that has been irrigated to maximize yields. An added bonus is that RDI is an exemplary way of conserving resources (water, and the added expense and energy to pump it) while also improving fruit quality. What RDI is not There is a real balancing act between lim- iting water to control excessive canopy growth and improving fruit quality ver- sus limiting water so much that you end up stressing the vines. When vines become stressed, the negative outcomes may include reduced yields, loss of basal leaves, delaying ripening of fruit (vines cannot photosynthe- 62 Wines & Vines AUGUsT 201 1 Between June and October, Regan Vine- yards utilizes deficit irrigation, giving 12 gal- lons of water to each vine, each month. size when they are seriously water stressed) and ripening fruit by dehydration. Dehydrated fruit may reach desired sugar levels, but you may find very unbal- anced wine chemistry (high acidity, low pH and harsh green tannins) and flavors described by winemakers as fruit cake, prunes and port. Some wine drinkers may actually enjoy these flavors, but if your goal is to make wines that are balanced and integrated (moderate alcohol, soft tan- nins, good acidity), this is not the kind of fruit you are looking for. Most certainly, this is not the objective of RDI. Spider mites are often encountered with stressed vines, particularly in warm cli- mates. Pacific spider mites thrive on dusty, drought-stressed vines, so part of an inte- grated pest management (IPM) strategy to avoid mite problems includes preventing vines from running out of water. By the time mite infestations are visible, you will expe- rience serious irreversible damage (for the n my most recent column ("Vine- yard Irrigation Strategies," Wines & Vines, June 2011 issue) I presented the concept of regulated deficit irri- gation (RDI), an irrigation strategy in which less water is applied than growing season) to the vine canopy, which will delay harvest and lower fruit quality. Winegrowers who are properly applying RDI hold back water so as not to encour- age vegetative growth, especially early in the season; then irrigation begins as vine growth slows. This creates an open, bal- anced canopy with dappled light into the interior of the vine and enough leaf surface area to ripen the fruit. (Berry size and fruit cluster weight are also reduced, but this will usually result in better quality wine if irri- gation is timed properly.) Irrigation ensures that the vine can carry on all normal physi- ological processes including photosynthesis and transpiration without being stressed. An effective RDI program Every vineyard is different due to the complexities of soils, rootstocks, varieties, trellises and climate, so you really need to start with an irrigation assessment of the specific block you are dealing with. Some things to know: The available water-holding capacity of a soil is based on texture (particle size) and effective rooting depth. Finer textured soils (clays and loams) are able to store water better than coarse-textured soils (gravels, sands, sandy loams) and make it a bit more difficult to initiate water deficits during the beginning of the season, when the soil has been recharged from rain. The coarser textured soils may require more irrigation (both volume and frequency) since the soil can't store water as well. You may need to initiate irrigation sooner when vines are planted in these kinds of soils. If the ef- fective rooting depth is shallow (less than 4 feet), you will also need to irrigate more compared to a vineyard planted in a soil in which the vine roots can extract water more deeply and thus have a larger reser- voir of moisture to draw from. Knowing the amount of water that your vineyard's soil profile can hold is very important in sched- uling irrigations. You can obtain general in- formation from the soil survey produced by the Natural Resource Conservation Service (NRCS). Local NRCS offices can produce a generalized soils map for your property and provide you with descriptions of the soils