One of our readers writes the following questions.
"Post harvest irrigation?? I’ve heard these policies;
1) A deep watering to get the soil moist below the roots to encourage deeper root growth and then leave them.
2) Just water to keep the vines “ticking along” but avoid tip growth. Our moisture sensors show that our deep post harvest water is depleted. Should we be applying another deep irrigation to encourage deeper root growth or just short irrigation when needed?"
Answer - Post-harvest irrigation is important but should be limited. If you irrigate too much the vines begin to grow again and use up energy / carbohydrate that would be better spent fueling budburst next year.
The aim is to avoid an unwanted flush of vegetative growth while ensuring that the vines. capacity to store carbohydrate is not affected.
I would think it is difficult to push water down below the rootzone at this time of year. If you applied an irrigation long enough it would also make the vines regrow.
Salts are best flushed from the soil in mid winter / early spring. Generally McLaren Vale region has enough rainfall for this to occur naturally however Winter 2006 and Spring 2008 were too dry for this to occur in some sites.
They also asked;
"Do we want to encourage deeper root growth? We got our best Cab quality this year. I think it was because the vines were less vigorous and offered more light for flavour development. I’m not looking forward to a return to heavy spring rains which send the vines into long shoot growth requiring summer trims.
I’ve enjoyed the control we’ve had over shoot length by applying water at the optimum times. I presume that deeper roots reduce control because they have access to the deeper soil moisture and keep the vines growing when, post fruit set, a vigneron may be wanting to stop the vegetative growth. What do you reckon about that?"
Answer - You are right, dry season's do limit growth. In some sites the reduction in vigour is a positive for grape quality, in other sites vines have become water stressed and this has contributed to a reduction in crop and I would argue quality also.
Over time vine vigour is reduced as older vines lose vigour. You may vine that during the next wet Spring your Cabernet is less vigourous than it once was.
As for the deep watering. Over time vines will explore every centremetre of soil available to them so encouraging deep root growth isn't a real problem. If water is available to vines and they can get to it they will grow there regardless.
For example in 1999 Langhorne Creek had a summer flood the soil around some vineyards was washed away and the roots were seen to stretch for dozens of metres along rows and headlands.
The only factors that limit vine growth are;
Compacted soil layers - these occur naturally, or by the activity of tractors / ploughs.
Chemical barriers - soil of high or low pH. Look for bleached white soil. Carbonate (limestone) is too alkaline for roots to grow into.
Hope this gives some insight into our thinking.