Thursday, September 3, 2009

What keeps vines dormant? - Q&A

I am curious about why bud burst seems to be happening earlier this season. Many thanks,

John Petters

Happy to help John,

Dormancy is a state of temporary metabolic inactivity or minimal activity. Vines generally go dormant in response to adverse growing conditions. We agree with your observation. Over the last three years bud burst has been seen to be getting earlier and is now occurring in late winter.
Healthy budburst in Shiraz - Sept 2009
We feel there are two reasons this is occurring. The first is warmer than average winter temperatures. The second is water stress after harvest.
Winter bud burst, or early bud burst is considered to be caused by over stressing vines the previous season. This is most commonly seen with weak vines near tree lines. Occasionally whole vineyards show winter bud burst triggered by warm winter weather. It is most common in varieties that experience bud burst at low spring temperatures- Chardonnay and the Pinot family.
Vines are held in dormancy by two forces - endodormancy (true winter dormancy) and ecodormancy (environmentally imposed dormancy). Endodormancy is caused by the vines internal clock – a balance of hormones and internal processes. Ecodormancy is caused by the environmental temperature, eg. when the Spring time weather reaches a certain temperature dormancy lifts and the vine buds burst.
If vines are over stressed after harvest then endodormancy can be weak and the internal balance of the vine unsettled. If the weather after harvest is warm then ecodormancy (environmental) can also be broken and the vine buds will start to develop. Any buds that burst will not grow through winter- they stay at 1-2 cm long until spring begins and normal bud burst occurs. In vineyards that have broken dormancy bud burst, flowering and harvest can be uneven.

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