Temperature and Plants

Bill Bampton explains the importance of temperature to the choice of plants we grow.

Temperature has a profound effect on plant growth. It is one of the most important schedulers of plant activity, of flowering, fruiting, budburst,

initiation of roots in a cutting and the falling of autumn leaves.

Although other limiting factors like rainfall, light intensity and soil types play their part, climatic temperature variation from the poles to the equator is the great arbiter of what plants grow where.

Killer frosts and searing heat set the limits of where plants will survive. That we grow cherries in Tasmania and mangoes in Queensland is largely due to temperature. As gardeners, understanding our local temperature range and seasonal fluctuations is the key to unlocking what we plant and when we plant it.

Hardiness and heat zones for perennials

The optimum temperature range for photosynthesis and respiration varies from plant to plant. Some have cell structures that tolerate freezing frosts, others can photosynthesise very efficiently above 30°C.

In nature, evolution has sorted out what plants grow where through thousands of years of adaptation to temperature and other environmental factors.

We gardeners have a vast array of plant material, of seeds from every climate and region. It all looks appealing, juicy mangos and Blue Himalayan poppies. We do not have thousands of years of trial and error to work out what to plant in our patch.

In the US, the Department of Agriculture (USDA) conceived a series of plant hardiness zones based on the range of average annual minimum temperature. This is the basis of the Cold Zones for Australia that The Diggers Club devised with the Bureau of Meteorology. However, the problem with looking at Cold Zones alone can be seen by glancing at the Diggers Cold Zones map.

Here you will find that Hobart and Alice Springs share the same Cold Zone (9b). Clearly plants in both areas will need to be frost hardy. Yet the difference in summer heat is profound. Only a fool or eternal optimist would base their Alice Springs garden on the plants that they saw thriving at the Botanical Gardens in Hobart.

This is why organisations such as Diggers and the American Horticultural Society have paired Cold Zones with Heat Zones. These 14 heat zones are divided according to the number of days above 30°C. If we take heat into account we now see that Hobart is in Heat Zone 1 and Alice Springs in Zone 12. Selecting your plants then involves checking that they fit with both your Heat and Cold Zone.

Microclimates

Remember that the temperature zones are simplifications. Some plants may be particularly affected by cold if it occurs early in winter before they have hardened off but tolerate later frosts or one day of frost but not repeated occurrences – all this detail is lost in the abstraction. If we come down from the broad scheme of temperature zones to our backyard, we get to the level of micro-climates.

We all know we have a sunny spot for sitting in winter and a cool nook for escaping summer heat. The western side of our house bakes in the afternoon sun, the south is shaded and cool, to the east warmth in the morning is tempered by afternoon shade, and to the north heat is maximised. As home gardeners we can experiment with crops outside their regular zone as our livelihood is not riding on their success.

Use microclimates to push the range of plants you can grow, experiment as they have done at the Royal Tasmanian Botanic Gardens growing bananas against a warm wall.

I have a long suffering coffee plant that was nearly killed by frosts when I moved to Melbourne’s western plains. Simply by moving it a few metres and under a north facing verandah it rewarded me with a handful of coffee beans! The other obvious way of extending the range of plants is with shelters, tunnels, cloches and shade houses.

Annual vegetables need accurate sowing temperatures

However, most of our vegetable crops are annuals and adapt to extremes of hot and cold by avoidance, adapting to particular seasons. Not being perennial plants we choose to sow them at the preferred temperature for germination – e.g. 10-15°C for cool weather crops and 15°C+ for warm weather crops (like tomatoes, zucchini and melons).

Seasonal fluctuations in temperature are the major determinate in sowing dates. The Diggers Club Sow What When chart gives detailed information on when to sow seeds for your particular climate. The broadest division of plants based on temperature is cool and warm season crops. Understanding this is fundamental to successfully time your planting.

Cool season crops are typically grown for their roots (carrots, parsnip and turnips), leaves (lettuce, cabbage, kale and spinach) or immature flowers (artichokes, broccoli and cauliflower).

Only a few cool season crops are grown for their seeds (peas and broad beans). These plants can endure short periods of frost once established. The low temperatures reduce energy use and increase sugar storage making them sweet. At the sign of warmth they tend to turn bitter or bolt to seed at the expense of leaf and root. Some, like cabbages, can be grown in warmer months but you will need to be constantly vigilant against water stress, insect attack and extreme heat.

Warm season crops are mainly grown for fruits (tomatoes, eggplants, capsicums, melons) and seeds (green beans, corn). They are tender plants – unless they are protected, don’t plant them outside until the threat of frost is over.

Warm temperatures mean these crops are photosynthesising and respiring at their peak so they need regular irrigation and fertilising to avoid stress and produce a good crop. It is especially critical with warm season crops to consider the number of growing weeks – that is those weeks above 15°C.

Check your sowing zone on the Diggers Growing Weeks map – Cool (9-20 weeks), Warm (21-38) and Hot (39+ weeks). While temperatures may be adequate to sow seeds you need to be sure that you have enough growing weeks to bring them to maturity.

This is critical when planting in cool zones. If you are planting tomatoes in Hobart be sure you select a variety and planting date that suits your available growing season. Think about extending the season by starting your plants indoors.

The big chill - number of chilling hours to fruit

Deciduous fruits add another layer of complexity. Many have evolved in climates with severe winters. If they were to flush with growth simply at the first sign of warmth, they could be fooled out of dormancy by an unseasonably warm winter’s day only to lose all their fruit when the cold returns. To avoid this, these fruits really need to know they have gone through the worst of winter before they break dormancy. To do this they need varying hours and intensity of winter chill (typically between 5 -10°C).

This is critical when selecting our fruit varieties in Australia where most of us experience warmer winters. If the buds do not receive sufficient chilling temperatures during the winter to completely release dormancy, the result is reduced fruit set and reduced fruit quality.

Diggers provides a range of "low chill" varieties such as White Satin nectarine. If you don’t get adequate winter chill you might even consider our Low Chill Orchard collection. Chill factors can also influence the breaking of dormancy in seeds adapted to cold winters.

Do some research and see if chilling may improve the germination rate of seeds you are sowing.

Climate change and your garden

The fact that planning our gardens is so reliant on predicting our climatic condition means that climate change will inevitably affect what we grow and when we grow it. For many, water restrictions have highlighted the changing rainfall patterns.

Yet changes to our temperature mean we have more than drought to worry about. Entire climate zones are on the move. The Arbor Day Foundation has created a dramatic animation of the shifting in hardiness climate zones in the USA from 1990 to 2006 (see more information and maps below).

Similar changes are no doubt happening in Australia. The Bureau of Meteorology is predicting that 2013 is on track to be the warmest year on record. Anecdotal evidence suggests plant confusion. A Facebook post from a friend in Adelaide shows a picture of fruit forming on her peach in August. Lesley from Kaydale bulbs in Tassie commented in an email:

"As to climate change, for us here in the mountains of Tasmania, gardening has always been a challenge and the catch phrase global warming had an appealing ring to it! We always thought that our area could do with a couple of extra degrees (Mum had always hoped to be able to have a fig tree and lemons).

We have found the reality to be not just so cut and dried, we are not getting the depth of snow but the mood swings of the weather are more pronounced. This year things have been a lot earlier, making mulching fun (hence tiptoeing through like a pixie!) with some of the spring bulbs a good week or two earlier."

I think Lesley’s point about the unpredictability of our seasons is important. Have you experienced some unusual plant behaviour triggered by unusual weather? If you have noticed any consistent shifts in plant behaviour, please let us know.

As gardeners it is vital that we share our experiences so we can make sense of fundamental temperature shifts that may affect the very what, when and how of gardening.