WHAT TO DO ON FROSTY NIGHTS by Brian Milligan
My best advice (for those of my age) is to put another blanket on the bed, pull on some bed socks, go to bed early and get up late! But dedicated growers who follow this advice still won't sleep well unless they've first made sure that their orchids are also cosy for the night.
July and August are probably the worst months for our orchids in terms of frost damage and it's therefore especially important to watch out for frost warnings at this time. Remember that the temperature in the suburbs of Melbourne is usually a degree or two lower than that forecast for central Melbourne. Cymbidiums can withstand temperatures down to freezing, perhaps even a degree or two below for short periods. But temperatures much below freezing can cause extensive damage to flower spikes. The unopened buds first turn brown and then black, before they eventually fall. Even spikes still in the sheath may be damaged. They may appear to be unaffected and still develop normally but when the flowers open they are often found to have pinched lips, sometimes with blackened margins. Australian native dendrobiums are also prone to frost damage, the leaves developing white areas and then falling from the plant. The large, delicate leaves of lycastes are also susceptible and even the fleshy pseudobulbs may freeze and subsequently turn into a rotten mass.
Fortunately most of us who live in the inner eastern suburbs don't usually experience particularly harsh frosts. Our suburban blocks are small and as a result our orchids are fairly close to buildings, either our own or our neighbours, which act as heat banks and therefore tend to keep their surroundings slightly warmer than otherwise. Those in low-lying areas or with open parkland nearby will be more likely to be frost-affected and should take extra precautions.
So what should we do to protect our orchids from frost? For most of us, a shade-house with a fibre-glass roof will provide all the protection we need for 99% of the time. Only on two occasions in the last 15 years have I lost cymbidium spikes due to frost damage and on both those occasions the temperature had fallen to -5°C and remained in that vicinity for several hours. The only way to counter such extreme weather is to provide some form of heat and air movement, for example with an electric fan heater. Alternatively, move all your plants in spike into the house or garage for the night - this is probably the best option for those with a small collection. But do it before you go to bed because it will be too late if you wait until morning.
Such low temperatures are rare in Melbourne's suburbs but we experience mild frosts every year, for which we should be prepared. Several of my shade-houses have shade-cloth fronts mounted on roller blinds. These are normally rolled up during the winter months but on frosty nights I unroll them to provide extra protection. If you have a large tarpaulin, its a good idea to pull it over the top of your shade-house, particularly if it doesn't have a fibre-glass roof. Draping sheets of newspaper over your cymbidium spikes is also a good idea – slightly warmer air is trapped beneath the paper. There is no wind on frosty nights, so there is no need to fasten the paper down - it won't blow away.
It's always colder at the perimeter than in the centre of your shade-house, so if possible move plants towards the centre. This is not possible in my overcrowded shade-houses and I occasionally notice frost burn on the leaves on my Australian dendrobiums located closest to the perimeter. Leaves touching the roof or walls and those close to the roof are also at risk.
Even those with heated glasshouses have to take care during frosty weather, because any leaves that touch the glass and especially the frame, if it's made of aluminium or iron, may be frost burnt. Also, I find that the orchids in my glasshouse need watering more frequently in very cold weather. The rate of evaporation of water from the pots depends on the temperature differential between the pot and the walls of the glasshouse and this, of course, is greatest in cold weather. It's obvious when you see all the condensed moisture on the glass on cold mornings and ask yourself where it has come from.
Some growers believe that they can counter the effects of frost by spraying their plants with water next morning until the ice has melted. The rationale is that if the ice is allowed to melt naturally under the influence of the sun's rays, additional heat is withdrawn from the plant's tissues (true), whereas the water sprayed on the plant provides the heat necessary to melt the ice. I'm sure that it's sound practice in the event of mild frosts. But at very low temperatures the sap within the plant has already frozen and the damage done, so I doubt if spraying with water helps under these circumstances.
For those members with a small collection, my best advice is to move your cymbidiums in spike indoors to a cool, unheated area. Don't bring them into your heated living room, because the sudden temperature change can sometimes cause bud drop. Of course, you must return them to their normal positions next morning (after the temperature has risen above freezing), making sure that the flower spikes face the original direction. If it seems like a lot of unnecessary work, just protect your best ones - at least you'll have a few plants for your winter and spring shows in the event of another super-frost!