How wet is too wet?

As the rain pours out of the sky today (30mm so far), with the ground already being so wet, pools of water are appearing everywhere, most alarmingly, in the back and front yards!!

As our own vineyard is on a slope, we don't need to worry about waterlogged soil, and many other vineyards in New Zealand are on free-draining gravelly soils, again, where water-logging isn't usually a concern.

However, in some cases, water can accumulate in the soil profile to above field capacity to total saturation, where no air spaces are left. If the water is free-standing for a while, it's not much of a problem for grapevines, but if it goes on too long, the roots will suffocate!

This is most likely to happen in the winter months, although in the spring if the snow in the mountains melts too quickly, you may get rivers overflowing their banks. In New Zealand, we can get flash floods in some places with heavy rainfall, and also flooding as a result of a lot of rainfall, saturated soils, particularly those that don't drain well (e.g. high clay content).

The difference between a decent-draining soil and a horrible-draining soil. These are two posts that I set in the ground yesterday - they're concreted in, but on the left the water has drained away, but for the post on the right the water is just sitting there. For the hole on the right, from about 20cm and down what I was digging out was almost pure clay! About 7m separates the posts.

In California and Oregon, back when I was studying on the West Coast of the US, there were several times of flooding, which meant some vineyards (or treefruit crops) were in standing water for weeks, or even months at a time.

So how long can roots survive under water? They need oxygen for the process of respiration, but in the winter, when the roots and above-ground parts of the vine are in dormancy, there isn't much activity in the tissues, so the use of oxygen is very slow. There can be oxygen in the water as well (this is how fish "breath" under water), and the roots may be able to take advantage of this, but the oxygen in the water that's near the roots, once the oxygen is used, the only way to replenish it is to have new water that has been exposed to air. Down in the soil profile, this doesn't happen so often!

So there are a number of factors that can contribute to the amount of time that roots can tolerate flooded conditions, but I'm of the mind that a few weeks shouldn't harm the roots or vines. In most cases, the water won't stick around that long.

During that all-important site selection process, these sorts of things should be investigated!

Meanwhile, I'll be hoping that the rain stops soon!!