Sap Bleed, In Deed
By Jessica Cortell
A common question I am often asked is, “Is it bad that my vines are bleeding?”
Referred to as “sap bleeding,” this phenomenon does not occur in all plants yet occurs in grapevines. The gelatinous clear liquid is often evident exuding from newly pruned wounds.
Sap flow is initiated when soil temperatures warm, stimulating the vines to start awakening from their quiescent dormant winter state. In the winter, the vines contain substantial stored reserves of starch and mineral nutrients. The reserves were replenished late in the summer and through leaf fall as the vines recaptured nutrients from the foliage. The warming temperatures trigger the starch and nutrients to be remobilized from storage cells into the xylem, the part of the vascular system moving water and nutrients up the vines from the root system — essentially, the water pipes of the vine.
The increase in solute concentration in the xylem from starch remobilization causes water to move into the roots as it naturally moves from a lower solute concentration to a higher concentration. As water moves into the roots, it creates positive pressure and pushes water up the xylem, resulting in sap flow or bleeding. Vines can generate enough positive pressure to move a column of water more than 10 meters above ground.
Is this a good or bad thing in terms of vine health? This bleeding, as concerning as it might appear, actually has a number of positive benefits to the vine and assists in preparing for spring bud break and new growth. Adequate soil moisture is needed for the vines to bleed sufficiently. While this isn’t a concern in the Willamette Valley, it can be a problem in dryer climes such as Southern Oregon and the Columbia River Basin.
One of the benefits of sap bleeding is how it reactivates the xylem by moving water up through the system. During the winter, air bubbles can form in the xylem. When root pressure pushes water up from the roots, it also pushes the air bubbles out of the xylem, creating an intact water column capable of transporting water to the buds. As the buds prepare for bud break, they take up water and begin to swell.
While this restores functional water transport, the vine’s other part of the vascular system, the phloem, is still inactive. The phloem is the system that moved carbohydrates throughout the vine as needed. As the nutrients stored in the vine’s bank (roots and trunk) are needed for bud break and all early spring growth, the vine needs a way to move these nutrients to the buds. Before and during bud break, this root pressure pushes water and remobilized nutrients up to the developing buds, helpful for consistent healthy bud break.
On the other hand, insufficient soil moisture, which may result in poor root pressure, has been associated with erratic bud break, stunted shoot growth, and/or cluster abortion early in the growing season as described by Markus Keller in the Washington State University 2012 Vintage Update (http://wine.wsu.edu/research-extension/2012/04/2012-vintage-update-13-april/).
This could obviously get the vines off to a very poor start early in the season if they are under conditions of low soil moisture leading up to bud break. Typically in the Columbia Valley, where soils are sandy loam and there is little rainfall, growers will refill the soil profile with water after harvest before the vines go dormant. Adequate soil moisture helps prevent cold injury in the winter as well.
However, if the soils have dried out prior to bud break (not likely in the Willamette Valley), a pre-bud break irrigation can be done. Keller’s recommendation is to apply irrigation to refill the soil profile 3 feet deep if you are not seeing bleeding and pruning wounds are dry.
So there you have it. No, they will not bleed to death! They actually need this transition in the spring to achieve a good start for another growing season.