Rootstock: the biggest mistake you can make in your vineyard

Soil chemistry is so complex that choosing a rootstock that is inappropriate for a specific soil chemistry condition is probably the most common mistake I see in practice. There are many ways to fail in this regard, but probably the most common one I see involves potassium, or more broadly, the balance among the major nutrient cations: potassium, magnesium, and calcium. While all three nutrients are essential for vine health and proper development of the crop, potassium is the one nutrient that can be problematic both when deficient as well as in excess. Potassium is a mobile nutrient in the plant and is involved with many plant functions, but perhaps the most critical role of potassium is in moisture regulation of the plant, specifically the action of opening and closing the leaf stomatal pores. Vines deficient in potassium do poorly in regulating their hydration. Severe deficiencies can cause basal leaf loss, and as the leaves senesce, they release what potassium reserves they have, which can end up in the fruit. Excessive juice potassium, which is commonly caused by excessive plant potassium levels, may also be caused by senescent leaves, which export their potassium to the vine and clusters before they drop off the vine.

Soils may be naturally high or low in potassium, but more importantly, the three main cationic elements (K, Mg, Ca) must be in balance for them to be taken up in proper amounts. With the exception of some volcanic soils that can have excessive amounts of potassium, soils in coastal California regions tend to be low to moderate in potassium. To make matters more difficult, many soils have elevated to excessive Ca or Mg. Central coast soils tend to be high in Ca while high Mg is found in soils up and down the coast. Serpentine-derived soils, common along the coast, are the extreme with Mg dominating the cation exchange sites of the soils. This can limit K uptake.

Choice of rootstock with respect to potassium is critical. Two common rootstocks used in consideration of potassium are 420A and 101-14, both very commonly used in coastal vineyards. 420A is a great rootstock – low vigor with a decent measure of drought tolerance – and it is very good at excluding potassium uptake into the vine. That can be very useful where soils are excessive or slightly so in potassium, and/or deficient in Mg/Ca that could lead to excessive K uptake. For this reason, it is often appreciated by winemakers who don’t want to see high K concentrations in their juice.

However, in low K soils or high Mg soils, very commonly found in our vineyard soils, severe potassium deficiencies can arise when using 420A rootstock. I’ve seen extreme K deficiency symptoms on 420A-planted vineyards, and their correction requires ongoing soil amending as well as fertilization, both foliar-applied and soil-applied. If not corrected, the severe K deficiencies can hamper fruit sugar accumulation and late-season leaf senescence may actually elevate the juice K, which is the opposite of what motivated the rootstock choice from the start.

On the other hand, 101-14 may be a good choice for soils severely deficient in K or in soils with elevated Mg, which would otherwise restrict K uptake. But, using 101-14 in soils with adequate K can cause elevated plant K and, hence, elevated juice K. Furthermore, 101-14 is one of the least drought-tolerant rootstocks in our arsenal of choices and has the frustrating tendency to shut down after experiencing water stress, dropping leaves, and tending to lag in its recovery. It also performs very poorly in heavy soils, especially those with cracking clays.

The rootstock 44-53 has the distinct feature that it excludes Mg uptake and is sometimes chosen for extremely high Mg soils, though I’ve also seen it used for soils that are simply elevated in Mg. For the latter, the rootstock is a poor choice as it can actually create Mg deficiency in all but soils of the highest Mg excessiveness.

Other rootstocks with a high-K tendency include St. George, 110R, SO4, 5BB, 3309 and Fercal, a rootstock suited for soils with high levels of free lime[i]. These should be avoided where soil K is high. On the other hand, rootstocks with a low-K uptake propensity include 1103P and 420A, already mentioned.