Which factors affect onset of bud break in grapevines, year-to-year?
What delays budbreak?
Do vines bud later in the season after their first crop?
Does water deprivation after harvest affect the subsequent season’s growth?
Is there any evidence that older vines bud later than younger ones?
These questions were recently occupying my mind (at least, the grape growing and wine making partition…). On March 23rd, I checked in on the vineyard and found that, except for 3 vines, none of my Aglianico had not shown any movement towards budbreak. Meanwhile, my 43 Montepulciano vines were all covered in green leaves:
In addition to robust budbreak, the Montepulciano had considerable sucker production:
But, as I said at the beginning, the Aglianico (the older cultivar, by one year) was significantly behind:
The Aglianico block had 3 vines with plenty of buds on their heads, 1-2 with scant buds, two or three with suckers. All others had not moved. There was no swelling or softening, let alone color change in the buds. The canes had wept/bled and had no sign of disease or being dead during pruning about a month prior. There were a few vines with small leaves at the ends of some canes at that time.
In 2011, my Montepulciano budded about a week before the Aglianico. Two or three vines were delayed but developed good growth later.
In 2010, the Aglianico had green buds or leaves between March 13 and April 10.
The Aglianico finally budded between my March 23rd welfare check and and my visit yesterday:
Still 3 or 4 vines remain dormant, but those will come around in a week or so.
Being very rarely satisfied with “Well, that’s just how it is” as an answer, I wanted to know why my Aglianico has come in so late this year. I want to know if there is an underlying problem and if I caused it and if I can prevent or ameliorate it.
First, some budbreak physiology.
Grapevines undergo a seasonal cycle of dormancy, new growth, fruit production and back to dormancy. In the context of this posting, I am most interested in what triggers a vine to come out of dormancy.
Soil and air temperatures are known triggers (of root and bud activity, respectively). But what are the pertinent physiologic processes in the bud and the cane?
Some basics from Wikipedia: In the Northern Hemisphere, budbreak begins in March, when daily temperatures rise above 10°C (50°F). Warming of the soil is associated with increased osmotic pressures in the xylem. There is an upward flow of water containing various cellular signaling molecules, hormones and other nutrients (which is why vines ”bleed” or “weep” during pruning). This process – or at least its rate as well as the rate and degree of its effect – may be dependent on the amount of water available in the soil.
In addition to growth-stimulating signals originating in the root system, the plant has other regulatory mechanisms in the canopy – in particular, the dormant buds. This is demonstrated by apical dominance - a phenomenon where apical buds (at or near the distal ends, “apex” of canes) “dominate” their neighbors and bud first while inhibiting (or suppressing) more proximal (or: “basal” – those closer to the head) buds.
The energy and building blocks for these initial processes within the young bud come from the stores in the root system. Once the new bud puts out leaves, those take over energy production, but it should be obvious that both of the above processes must take place fairly close to each other in order for the plant to grow.
What happened in my vineyard this past winter and spring?
Most literature deals with budbreak occurring too early (document courtesy of Dr. Jim Wolpert, UC Davis) – for understandable reasons. In my web search, I found no focused treatments on “preventing and/or dealing with delayed budbreak”.
My first thoughts were to trace back over the past year to identify anything I may have done differently with the Aglianico block.
Looking at the diagram, above, I see four potential contributory (if not determining) factors. These are: pre-dormancy water deprivation, effect of delayed pruning, effect of first crop and the age of vines. I searched the web for abstracts and articles which would shed some light on this issue.
Post-veraison/pre-dormancy water deprivation.
Because of a series of events in my personal life last year, I did not irrigate for 2 -3 weeks after veraison. The vines went through some degree of water deprivation after fruit maturity and before dormancy.
Rainfall last year – as in the rest of the state – was considerably lower than in the previous years. The 2011 winter was not only drier, but warmer – by virtue of average, as well as the lowest temperatures:
This represents an upward creep: The last three springs have also been increasingly drier. 2012 was a bit warmer than 2011. The same trend exists between 2011 and 2010. Again, this is based on average and lowest temperatures recorded.
While water deprivation/stress seems tempting as a causative factor, some investigators suggest that water stress tends to push up budbreak – at least in one Japanese variety (abstracts here and here). The explanation for this being that inhibitory substances (particularly, abscisic acid) are more concentrated in dormant canes of water-deprived/stressed vines.
One could argue that the Aglianico vines get slightly more water because they sit downhill of the Montepulciano. So, as the irrigation system drains, each lower row should get a bit more water. Similarly, as rainwater flows downhill, the soil in the Aglianico block should, theoretically receive more water. Budbreak patterns in the previous years do not seem to support the notion that this difference has a noticeable or meaningful effect.
My two varieties have been affected differently despite receiving essentially the same amount of water. This potentially eliminates water deprivation as the culprit, but the caveat here is that the Aglianico had a larger canopy in 2011 and it bore fruit. Thus, it had a larger water demand, so the possibility still exists that its soil my have been more depleted of water.
If budbreak requires the presence of regulatory substances produced or stored in the roots, less water in the soil would mean lower water (hydrostatic or osmotic?) pressure in the xylem to push these to the end of the cane (or spur). However, I was not able to find any mention of this mechanism so my speculation may be entirely off-target.
Finally, I considered the possibility that this block was shocked if not hurt by lack of post-veraison irrigation. I cannot find any way to verify or dismiss this notion. It is, however, less likely because Paulsen rootstock (to which my Aglianico is grafted) is considered drought-resistant.
Effect of pruning on budbreak.
As the timeline schematic above shows, I pruned the Aglianico about two weeks after I pruned the Montepulciano. Both blocks had some apical leaves when I started to prune the Montepulciano. There is evidence that pruning has an effect on onset of budbreak. The effect is significant enough that delayed pruning is employed by growers in cold climates as a method of reducing frost damage.
This method takes advantage of the phenomenon of apical dominance to delay budbreak (hopefully, after the risk of frost is greatest). The time needed for the effect of inhibitory auxins to dissipate (and the remaining, basal, buds to start growing) is the time “bought” when the intent is to delay budbreak past frost risk.
Effect of first crop on subsequent budbreak.
It is natural to suspect that the processes of fruit production and ripening might deplete some of a vine’s stores and reserves of energy and nutrients as well deprive the plant of the opportunity to produce and store budbreak-modulating substances. However, it is a big hypthetical stretch if this leads to later budbreak in the subsequent year I have not found data to support this idea.
Yet, the second of the two Japanese studies I cited above (the one addressing water deprivation to induce a second crop in one year), seems to contradict the intuitive notion (that fruit bearing strains a plant’s resources) as the authors of this Japanes study state there was no difference in quality of fruit. If fruit quality (it’s unclear what indices of quality were used) is not affected, then it’s conceivable that whatever resources are needed to induce budbreak are not depleted by a crop. The time of budbreak is not likely to be affected by the vines’ yielding their first crop in the preceding season.
Effect of vine age on budbreak onset.
I have heard it said, anecdotally, that young vines go through budbreak earlier than mature ones. I am not sure who said this, when they said it or in what context, but the general idea seemed to be that in the first few years, vines can bud earlier than they do after a few years of growth. I have not found data to support this yet. My curiosity on this point remains unsatisfied.
My initial thoughts, as I saw the Aglianico block on the 23rd was that it was unlikely that a disease or gophers had wiped out over 90% of the block in one season. I knew budbreak can be delayed for about a month in some cases. I suspected it would come, eventually.
I wanted to know, however, what had caused the disparity between the Montepulciano and Aglianic blocks.
Having looked at the four possible causative factors (pre-dormancy water deprivation, delayed pruning, first crop in the preceding season and the age of vines), the factor most supported by evidence as most likely to delay budbreak is delayed pruning. This is a reliable and frequently used method of delaying budbreak in cold climates.
Finally I contacted Tom Marble who had planted the three Aglianico vines I had given him. These vines, which I’d kept in pots in my back yard were pruned before I pruned the Montepulcianos in the Vineyard. Their advanced growth, relative to my vines (same scion clone) lends further support that the disparity between budbreak in my two blocks was most likely due to the timing of pruning.