A short blog as I have been preoccupied with everything but gardening for the last few weeks! We returned from a short trip to Far North Queensland on Tuesday night last week, and were plunged into freezing temperatures: a shock after a balmy 27° day in Townsville! I couldn't understand why I felt so cold that night, but the next morning discovered it had been the coldest Sydney June night for more than 60 years.
My garden experienced quite severe frost damage that evening. Frost tends to occur on clear nights, when the lack of cloud allows heat to quickly radiate from the ground, causing a significant drop in temperature. Frost occurs when the temperature falls to below freezing, as a thin layer of moist air near the ground forms ice crystals without first condensing into dew. The crystals coat any cold surface, including plants. During a severe frost, the water within the leaves and stems of plants will freeze, causing cell damage and a blackening of the leaves.
Many of my Plectranthus were burnt, as were some of my coleus, warm climate shrubby Salvia and Iresine plants. From other gardeners, I have heard of shrub and cane Begonia, Pentas and Heliotropium being knocked. For once I was glad of my standard edict on pruning these sorts of plants: not to do it before around mid-August. Even though this does mean a couple of months putting up with a rather straggly garden, the dishevelled summer growth provides protection for the plant in the case of a night such as last Tuesday. If you also have plants which have been burnt by frost, don't cut off the blackened leaves, as this will only expose the lower growth to the same problem if another frosty night occurs this winter. With July being the coldest month of the year here, this remains a possibility.
It is a reminder that although it sometimes feels as if we live in a subtropical climate and can grow many frost-tender plants, we are not really in that zone and must be cautious of what we choose for our gardens. One way to reduce the problem can be to cover susceptible plants with old sheets or plastic if frost is predicted. A canopy of trees in a garden seems to reduce the likelihood of frost to some extent. It is also a good idea to take cuttings of cold-sensitive plants in autumn and keep them under cover during winter, in case you lose the main plant due to a bad frost.
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