Frost is when a solid surface is cooled below the dew point of the adjacent air, so frost can happen at any temperature but will normally only happen at temps below 3 degrees C.
Damage is caused to our plants when the water in the cells expands and breaks the cell wall, we also get damage from sunburn and sudden heat changes when the sun hits plants which are frozen.
To bring a frost into perspective think of your car sitting in the sun and how hot it gets, it has the ability to absorb a lot more heat than most items. Remember how hot your car get’s in the sun? In absorbing the heat quickly it also loses heat quickly, so on a cold night whatever heat has been accumulated through the day is lost very quickly.
So the car’s temp of a night can drop to well below freezing without the night time temp dropping below zero. That’s why you can have ice on your car at temp’s above freezing.
The other way to understand frost is as heat is lost from the surface of the garden it travels upwards.
On clear, still nights the air travels upwards and is replaced by cold air naturally occurring in the upper layer of our atmosphere. This cold air slowly descends into our garden forming in the lower areas first, once it fills the lower areas it moves to slowly fill the upper areas, like filling a dam.
On cloudy nights the heat is reflected back down to our gardens, while on a windy night the air is blown around and constantly mixing with other layers, keeping our nights warmer. Your garden is the same, the plants and gardens ability to absorb and retain heat are in direct proportion to its ability to handle frost.
Tips for mechanical frost protection
- Plant frost hardy plants. It sounds stupid but sometimes the simplest concept is the best.
- Every metre of height will equate to around 1 degree of protection, so the higher up the hill you plant your plants the more protection your plants will have. The taller the tree the more protection it has.
- Keep plants like Agaves and Sanservia dry. If they don’t have much water stored in their leaves they won’t have water in the leaves to freeze, expand and crack the cells in the leaves.
- Houses radiate heat so plant your frost sensitive plants close to your house (1m).
- Plant windbreaks, trees trap heat and slow the movement of cold air across your property. Large open areas are Jack Frost's friend.
- Cover the trunks of smaller trees with a binding of hessian, this will stop the trunk from freezing before the tree gets a chance to grow and produce natural corky bark.
- Cover plants late in the afternoon with old sheets, and then a layer of plastic or a tarp, remove the tarps and sheets in the morning.
- Keep lawns trimmed and short, short grass allows sunlight to hit the ground which allows the ground to heat more during the day and hence has a larger source of latent heat to release at night.
- Trees act like frost umbrellas, so plant your frost sensitive plants under tree cover.
Tips for chemical frost protection
- Use Seasol regularly through the colder months, every 7 days. Don’t use Seasol powerfeed as it has nitrogen in it and it will encourage growth – we don’t want growth, we just want protection. Normal Seasol will increase the salt levels in the plant and increase the thickness of the cell wall, both these factors will increase the plant's ability to handle cold by decreasing the temp that a cell will freeze. Once the frost Seasol has abated switch to Seasol powerfeed.
- Use products like envy or wiltnot the day before a cold event. The Internet will tell you an accurate 7 day weather forecast. Plan ahead and spray your plants with these products before the event. These products are a water emulsifiable polymer concentrate which forms a semi-permeable, biodegradable film which assists in the reduction of transpiration caused by climatic extremes (i.e. a coating that slows the plants ability to lose water in extreme climates)
- Spray the plants with water before the sun hits them, this will help the plants to heat gradually. The sun on a clear frosty morning will heat the plants from minus something to 15-20 degrees C in minutes, which is not good for plant health. Remember on a cold night to leave your hose laying straight and flat and empty of water, a frozen hose is of no use to you.
Each practise you put in place will only help your plants in increments, so the more ideas you implement the better. The best tips we can give you is to monitor your local weather and prepare in advance for the cold nights ahead.