Wind challenges for BoP orchards
New Zealand has a temperate maritime climate that offers unique opportunities for growing produce. But being temperate and maritime also has drawbacks. The consequent variability of climate can provide challenges for growers.
Too much rain at times, which has certainly been an issue for many North Island growers this season, and occasionally too little rain. Temperatures are fairly reliable and in the ‘Goldilocks range’, i.e. not too hot and not too cold, because we lack the extremes of temperature seen in more continental climates. Growers have also generally been careful to select crops suitable for the temperatures in their area.
The other thing that comes with a temperate maritime climate is wind. Most orchard blocks are well sheltered to protect trees and vines from cane or branch breakage and fruit marking. Many growers, however, know that one side of a block, or transect through a block, does get wind damage as land drops away to one side. Higher parts of the block may not be adequately sheltered from the prevailing wind, or wind comes through gaps in the shelter which are there for access or to ensure views from the house are not compromised!
Bay of Plenty growers may have noticed this last season that wind damage was particularly bad where shelter is not ideal on some parts of their orchards.
Wind is measured as wind run and wind gust. Wind run is measured as kilometres per 24 hours. It is the distance the wind travels, in one place, that day. It is frequently measured using a cup anemometer. See picture 1.
As the wind blows the cups turn; the wind speed can be derived by counting the number of full rotations of the cups.
Because wind changes direction during the day there is no directional data available with the wind run measurement from NIWA. It is the wind gust data from NIWA that provides us with directional information. As gusts occur within the stream of the prevailing wind, not against it, we are advised of the direction of the stronger wind on that day. All data used to write this article is provided by NIWA via the CliFlo website and is based on Tauranga data.
The Tauranga weather station at the airport, near the harbour, records higher wind speeds than the Te Puke weather station in the midst of the Te Puke kiwifruit growing area. The Te Puke station is better sheltered from the wind, though located in a relatively open area. There are many orchards located on the fringes of Tauranga harbour and further down the coast to Maketu and beyond, so using Tauranga weather station data is valid.
UNITS OF WIND MEASUREMENT
Wind is measured by a number of units. The earliest scale, still used today to describe wind speed, is the Beaufort Scale. The Beaufort Scale was based on observations of sea conditions and how those related to sailing ships in the early 1800s, ranging from so little wind it was hard to steer the ship, to winds that would rip the sails. It was later extended to include observations of land conditions. Ranges expressed in metres per second (m/s) and kilometres per hour (km/h) were added for each of Sir Francis Beaufort’s 13 described wind categories.
For the purposes of this article I have divided wind run data into units above or below 500 kilometres per 24 hours. A wind run of 500km/day equates to an average wind speed of 21km/hour which is described on the Beaufort Scale as being at the lower end of a moderate breeze. A moderate breeze ‘raises dust and loose paper; small branches moved’. If small branches are moved then the rapidly growing canes of kiwifruit vines with their large leaves will be quite mobile. Also 21km/h is only the average wind speed, there is considerable