Saturday, July 11, 2009


“Would like to know what can we do to protect plants from frost? I heard there is a spray for leaves, but i've had no luck finding it yet. Is there such a thing?
Thanks for any info.
Best Regards, Patricia”

I recall mornings where the water froze in the nursery pipes but I’m sure we haven’t seen the like in the past 15 years. It can however still get very cold some mornings, cold enough to damage some of our favourite plants. Melbourne is a pretty safe horticultural environment when it comes to frosty mornings but there are definitely parts of Victoria that frost can be a real issue.
Frost is a weather event like extreme heat or hail. We deal with it relatively rarely say two or tree times each season, but we spend an enormous amount of energy and effort preparing ourselves for these few events. At this time of year we watch out for high pressure systems and those beautiful clear evenings when the temperature drops suddenly, it’s a fair guess that we are in for a cold night. It is important to understand that frost is not cold air, cold dry air just helps produce the frozen frost crystals. It is also important to remember that the very cold air that produces old Jack Frost is heavier than other air so it pools in low, sheltered places, not on hill tops or slopes.
So what do we do to protect our plants from Frost? Well let’s start by not growing plants that are severely damaged or by growing them in a protected environment. I have never forgotten the heartbreak of having been forced to move Tomatoes and Marigolds outside early one October because we had completely run out of protected space. October! We had the lightest of frosts but it was enough to turn all of those beautiful plants to a very ugly, black, melted plastic constituency.
Protection is the other critical frost protection factor. A shade cloth cover can be all that is needed. We have when desperate, risked the physical damage of simply laying spare pieces of shade cloth (Hessian or a sheet would work perfectly well) directly on exposed plants. Of course we have green houses and shade houses for protecting plants. Shade from trees is just as effective as is the eve of a building or the radiated heat absorbed by a north facing wall.
If physical protection from frost is not available, water can be very valuable. My understanding is that the “burning’ symptom we see is the result of the plants cells having been frozen. When the cells of frost sensitive plants are frozen they collapse and die. This of course happens in the coldest hour before dawn. Applying water can have a number of effects. Primarily the water is warmer than the freezing air. This of course can change pretty quickly but the theory is that water actually releases a little heat as it converts to ice but this is of little value in our situation.
The other advantage of watering on frosty mornings is that the activity actually stirs up the cold air, mixing it with marginally warmer air. This is a theory used commercially where large fans, even helicopters are used to circulate air over crops on frosty mornings. Ok so this is probably not feasible at home.
There are chemical solutions to the frost problem that are used quite widely in the nursery industry so I assume they are reasonably effective. Copper based sprays like Bordeaux mixture are believed to strengthen plant cells, protecting them from frost damage. Yates produces DroughtShield which is widely available in good garden centres and is apparently quite effective. Envy is the most commonly used product in the trade but I am not sure if it is available in home garden packages.

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