Cultural controls to reduce spring frost risks – Fruit & Vine

Aside from careful site selection, one of the biggest ways to manage frost risk is to ensure good airflow around the vines, reducing the risk of cold air becoming trapped and frost forming, Chris says.

Cold air naturally sinks to the lowest point, so dips or depressions in the ground, and the base of slopes, will be at greatest frost risk. “It is therefore vital to maintain air drainage holes in hedges and undergrowth at the bottom of slopes, so cold air can escape. Experience shows that running water near these holes can help draw cold air away more effectively.”

Keeping boundary hedges trimmed, along with other vegetation, such as cover crops or long grass in alleyways, also aids airflow, he adds.

Weeds inhibit air movement too, potentially increasing frost risk by trapping cold air at night, so maintaining a weed-free area beneath vines is advised. Bare soil is also more effective at absorbing the sun’s heat during the day and radiating it back out at night, raising the temperature around vines slightly to stave-off frost.

However, Kent-based Hutchinsons agronomist, Rob Saunders points out that leaving soil bare overwinter conflicts with the principles of healthy soil management, so it may be preferable to retain some weed cover in the under vine strip until closer to the main frost risk period.

“A balance must be found though, as we cannot expect a single herbicide application to control large, well developed, weed populations in one hit, therefore a staged approach using different actives and modes of action may be required on particularly weedy sites.”

Glyphosate is one of the main herbicides available to growers, but Kerb (propyzamide) offers an alternative mode of action, which is good for managing long-term resistance threats, he notes. “It is also slower-acting, taking several weeks for growth to die back after treatment, thereby providing some soil protection and substrate for soil organisms during this period. Remember, Kerb has a limited application window of 1st October to 31st January.”

Delaying bud break by pruning later can be a useful option for mitigating frost risk, although there are practical limitations to doing this across large vineyards, Rob says.

“Growers should critically appraise sites and prioritise the most frost-prone areas to prune last.”

Leaving extra upright sacrificial canes can be beneficial in high-risk areas, he continues. Doing so gives more material to select from should frost damage occur, and takes advantage of the vine’s apical dominance, whereby the distal buds on the upright cane tend to break first, fractionally delaying the emergence of buds on the other cane that has been tied down. “The delay may only be marginal, but it could make all the difference.”

Retaining more buds than vines can support gives similar flexibility in case buds are lost to frost. But for both options, if little or no damage occurs, it will be necessary to go back to prune or rub-out excess buds and/or complete tying down once frost risk has passed to manage canopy density, airflow and crop load.

Chris Cooper says delaying tying down, or bowing the cordon to minimise bud contact with metal wires can also help. “Wires conduct heat and cold, so generally the bud closest to the tie on the wire gets frosted first if bowed, whereas all buds can get scorched if laid flat.”

He reminds growers that the height of the cordon above ground level will affect the likelihood of frost damage occurring, with lower cordons more at risk from ground frost. “Individually, these are small things, but they are all worth considering.”

From biostimulants to polymer sprays, there are many products on the market that claim to provide some level of frost protection, but both Rob and Chris are cautious about how much they can be relied upon.
Applying copper to vulnerable shoots before the main frost risk period, for example, is thought to reduce the presence of ice-nucleating bacteria (e.g. Pseudomonas syringae) on buds, or foliar tissue, thereby reducing the likelihood of frost forming. “While there is some evidence to support this, there’s little from UK vineyards,” says Rob. “Other literature suggests that keeping grass short can also reduce the amount of Pseudomonas syringae that makes its way onto buds.”
Experience with spray-on acrylic polymers (e.g. Antistress) and hydrophobic particle films, that form a physical barrier against frost, has also seen mixed results. “While they could potentially help, we must not expect too much.”

Both experts think the glycine-betaine based product, Lalstim Osmo, looks promising though. The natural extract is an osmoprotectant that increases the strength of plant tissue and boosts the circulation of sap flow, water and nutrients, thereby offering some protection against light frost when applied 24-48 hours before frost is due.

Similarly, foliar-applied biostimulant feeds, such as Maxicrop, Kelpak or Cultigrow CBL, may help reduce frost damage once growth begins, by improving the concentration of sugars and salts (solutes) in and around the buds, thereby lowering the freezing point of the solution within those cells.

If frost damage has occurred, they can also help plants repair damaged tissue and/or stimulate replacement bud growth, Mr Cooper notes.