Frost!

Sunday, 04 July 2010

Birdbath broken by the frozen water in its bowl during the frost last week, in the garden of Julia Price in Sydney.

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.

Reader Comments

  • By therese 2119 Monday, 05 July 2010

    Thanks Deirdre - I didnt know about retaining the blackened leaves for protection so will now use this information in my garden - I cant believe that our front lawn survived the frost without apparent damage, although it will start to brown naturally soon enough. Therese

    I do think it is best to leave the blackened leaves on; otherwise if another frost comes, the remaining leaves will burn and the plant will be more vulnerable. We may not get such a bad frost again this winter but we never know. Deirdre

  • By jan 2072 Monday, 05 July 2010

    Hi Deirdre, Those cheeky oyster plants have protected a lot of sensitive plants for us during the frost. We are off to north Queensland in the Whitsundays so hope to leave the cold behind until August. Jan

    Have a great holiday, Jan. North Queensland is perfect at this time of year. Deirdre

  • By Helen 2154 Monday, 05 July 2010

    Hi Deirdre, and welcome back to sunny Sydney! My carefully chosen and nurtured collection of Begonias that are here and there in my garden are a sad sight. Also my beloved Streptocarpus. I just hope there are live roots underneath to delight me in spring. What is your photograph? Regards, Helen.

    Sorry to hear about your frost damage. Once the warmer weather arrives in mid-August we can prune off all the blackened leaves. The photograph was taken by Julia Price of her birdbath last Wednesday morning: the water froze and caused the bowl to break. The ice can still be seen in the rest of the bowl.The temperature was minus 5 degrees in her garden. Deirdre

  • By margaret 2122 Tuesday, 06 July 2010

    Thanks for your concise explanation of frost. Also had damage - some shrub, rhizome begonias, front half of the plants, coleus and nasturtiums. Sprayed, too late, with drought shield and seaweed solution. Prune later, end of August.

    Thanks, Margaret. I did not know you could use Droughtshield against frost. Hopefully, we will not get any more nights like last Tuesday this winter. Deirdre

  • By Sue T. 2566 Friday, 09 July 2010

    Hi, Like you Ive been enjoying warmer weather up north and arrived home from Darwin yesterday to find a lot of my Salvias very badly burnt. Im hoping some will regrow but many look very doubtful.

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