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Managing fire damaged grapevines

Grapevines react to fire in several different ways and the extent and type of damage determines the best approach to treating the vines. Some effects of fire are obvious, such as dehydrated leaves and burnt bark, but other damage can occur below the surface of the plant, i.e. radiant heat causes damage or death to the plant without obvious external damage.

 

Checking for damage

Most internal damage can be readily checked within a few days of a fire event but the full extent of damage may not be obvious until the following season. The cambium layer is a narrow band of tissue just under the bark that produces the vascular system of the plant. The vascular system conducts water, nutrients and hormones through the vine and new tissue is produced every year. Once this layer is killed there is no further vascular tissue produced and that part of the vine dies. Being obscured under the bark it is not always possible to determine the health of the cambium layer from external observation.

There are two different ways to check the viability of the internal tissue. The first one is non-destructive and each plant can be checked. Using a sharp knife, make a small cut into the wood, much like that used in T budding, to reveal the health of the cambium layer and vascular tissue. If the tissue is moist and creamy white perhaps with a greenish tinge, then the cambium is still alive and the vine has a good chance of recovery. If the cambium is dry and brownish in colour then it is dead and that part of the vine will not grow. Fire may damage one side of the vine but not the other so it may be necessary to check a few sections around the trunk.

 

"Image: Figure 1. showing healthy, damaged and dead tissue in grapevines."

From left to right, healthy, damaged and dead tissue in grapevines.

The second method of checking viability is destructive and involves cutting completely through the vine and staining the tissue with methylene blue. Live tissue stains bright aqua-blue and dead tissue stains a brownish-blue colour. The latter technique is more useful for assessing the viability of a large area of vines being considered for removal.

 

Variation in damage

Image: Figure 2. showing where the fire front moved from right to left across a block of vines.
The fire front moved from right to left across this block of vines.

Dried up leaves are an obvious symptom of fire damage. If the vine only has partially singed leaves then there is unlikely to be any effect on the viability of buds along the canes and they will burst and produce crop in the next season. Vines with all leaves dried up reflect a higher level of radiant heat and this may reduce bud viability for the next season. Checking bud viability along the cane in early winter will help determine a pruning strategy (e.g. short spur, long spur or cane pruning) that will produce adequate regrowth and crop. Vines closer to the direction the fire comes from will be more severely damaged than vines further into the block.

Severe fire will scorch the trunk and show blackening on the bark. This reflects quite intense heat damage and the trunk should be closely checked for viability of the vascular tissue under the bark. Vineyards with straw mulch along the vine row are more badly affected by fire. Mulch that burns right around the vine is likely to completely kill the internal tissue of the vine at ground level. There may be regrowth from suckers that can be trained up as a replacement vine but where the lower trunk of vines has been completely disbudded (e.g. with benchgrafted vines on rootstock) there will be no regrowth and the vine will need to be replaced. Vines that are under stress or have small trunks suffer more from fire damage than well watered vines or vines with larger diameter trunks.

Radiant heat damage needs closer checking because there is no obvious external blackening on the bark. Only the part of the trunk facing the direction the fire came from may be damaged and the sheltered side may still be alive. The vine may well survive and produce new top growth but there will always be a strip of dead wood in the trunk, which may become susceptible to wood rots and termites. If only a small section of the trunk is alive it may be a better strategy to replace the vine.

Not all vines that regrow after a fire will survive in the long term. Some collapse a month or two after regrowing, others collapse in the following season. It can be difficult to exactly determine the extent of damage on every vine soon after a fire.

 

Responding to fire damage

Vines with leaves that are partially or totally scorched should have the crop removed to eliminate competition for water, carbohydrates and nutrients. Grapes may well have picked up a smoke taint and rendered unsuitable for winemaking anyway.

Plastic irrigation lines and above ground controllers can also be destroyed by fire. It is vital the irrigation infrastructure is replaced quickly (if rain doesn’t fall) particularly if the soil is quite dry. Water should be applied to match the vine growth and not overdone such that it saturates the soil and reduces root function.

The timing of the fire damage also influences the response to fire damage. Fire damage before fruit set may allow enough time for regrowth to harden off before the frost period in autumn such that the regrowth can be used for pruning for the following season. However the fruitfulness of that regrowth is likely to be low. Fire damage after veraison does not provide sufficient time for regrowth to harden off and provide adequate pruning material for the following season. In the latter case the growth before the fire will be adequately hardened and fruitful to be used for the next season. Any inflorescences on regrowth should be removed to maximise carbohydrate production going into vegetative growth. There is insufficient time for reproductive growth to produce a viable crop and it is important to maximise carbohydrate storage for vine growth in the following season.

In most cases, and certainly when fire damage occurs after veraison, the vines should be left to regrow during which time viability assessments can be made to determine the best strategy for dealing with the vineyard. With a high degree of vine death or damage it may be preferable to remove a complete block and replant. With lesser degrees of damage it may be worthwhile replanting those vines that die but it is often difficult to manage blocks with mixed ages of vines.

Growers have considered pruning vines back after sustaining fire damage. This may be worthwhile where fire damage occurs early in the season say before fruit set. Pruning after fire damage from around veraison onwards cannot be justified because the regrowth has a limited period in which to grow and mature before the frost period.

Experience has shown that vines can take 2 to 3 years to get back into full production and some vines can still collapse after showing initial signs of recovery. The amount of work required to rejuvenate a mix of dead and sick vines needs to be weighed up against a total replant of a block. It may also present an opportunity to replace under-performing varieties or to change over to grafted vines on phylloxera, drought or salinity tolerant rootstocks.

 

Further reading

Scarlett N, Needs S and Downey MO (2011) Assessing vineyard viability after bushfire. Aust. NZ Grapegrower Winemaker 564:21-25.

Whiting J (2011) Grapevine recovery from fire damage. http://www.awitc.com.au/posters/downloads/pdfs/AWITC_Posters_148.pdf

Whiting J (2012) Recovery of grapevines from fire damage. Aust. NZ Grapegrower Winemaker 580:25-31.

Wilkinson K and Collins C (2010) Grapevine recovery following fire damage. Aust. NZ Grapegrower Winemaker 556:42-43.