#5 - Mar 2017 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz
TEMPERATUREMarch/April: Early-/mid-autumn. Late harvest, scarlet runners, grapes, olives. A horrid summer. A cold autumn. Plants go slow in the cold. Why? By how much? Does it matter? What can you do?
Plant growth & temperature: All biochemical reactions go faster as temperature rises. Plants live at air temperature, so grow faster in warm weather and slower in cold weather. Plant growth about stops at 10°C.
Days: Air temperature is highest in mid-afternoon and lowest at dawn. In between, it changes like a smooth wave. We usually measure just the maximums and minimums. Midway between these is the average. Yesterday, the max was 15°C and the min was 11°C, so the average was 13°C.
GDD: Plants grow at a rate proportional to “average temperature, minus 10°C ”. Yesterday’s growth was only about 3 units (average 13, minus 10 = 3). If the weather had been what we’d expect for March – say, max 18°C, min 14°C, the average would have been 16°C. Then plant growth would have been about 6 units (average 16, minus 10 = 6). Twice as much. We call these units growing-degree-days (GDD).
Seasons: The growth you’d expect in a month is the total of all these daily GDD growths. For Eastbourne, through the year, we’d expect: Jul (0), Aug (6), Sep (48), Oct (87), Nov (135), Dec (195), Jan (237), Feb (240), Mar (198), Apr (132), May (75), Jun (15). So, plants grow a lot in summer, a bit in spring and autumn, and not much in winter.
Regions: The annual GDD for Eastbourne is about 1300 (adding up all the monthly GDD). The NZ climate is warmer north and colder south, so plants grow much more in Kerikeri (2100) than in Invercargill (500). Meanwhile, Auckland is (2000), Tauranga (1800), Napier (1500), Christchurch (1200), Ashburton (1000), Timaru (800) and Balclutha (600).
Maps: Other things affect air temperature too, such as height above sea level, closeness to mountains and so on. Maps can give you annual GDD for every location in NZ – drop me an email and I’ll send you one for the south North Island.
Move north: If you want more plant growth, move north. If you want to stay in Eastbourne, grow plants by north-facing walls (masonry holds warmth) or under glass or plastic (greenhouses, cold-frames, ½-bottles). And please stop whinging about the weather…___________________________________