ever wondered how farmers know when to grow their crops and when to harvest them? how about which crop can or cannot be grown? well, thats there Accumulated Heat Units, otherwise known as Growing Degree Days and Cumulative Heat Units, come in.
As with all living things, plants are reliant on enzymes to catalyse almost all of their metabolic pathways. the rate of reaction of an enzyme catalysed reaction is highly dependent on temperature- low when cold, increasing with temperatures below the denaturation point. below a certain Base Temperature, enzyme action is so low that the plant 'shuts down' and there is little growth. because of this, we can use the amount of time a plant has experienced above its base temperature to calculate it's growth stage. the method to calculate AHU is as follows:
mean daily temperature = (highest + lowest)/2
daily heat units = mean daily temperature - base temperature
accumulated heat units = sum of daily heat units over period of cultivation
to illustrate this let say we are calculating AHU for pea plants. Yesterday's daily high was 28ºC and its low was 17ºC. Today's daily high was 30ºC and the low was 16ºC. peas have a basal temperature of 20ºC
our mean daily temperatures are as follows:
yesterday: (28+17)/2 = 22.5
today: (30+16)/2 = 23
our Heat units are as follows:
yesterday: 22.5-20 = 2.5
today: 23-20 = 3
our AHU would be the sum of both days, or:
2.5+3 = 5.5
but how do farmers use this? well, each plant requires a certain amount of heat units to complete its growth and for maximum yield. by comparing this with historical data for available heat units in a season, farmers can come forecast the time they need to plant so they will be able to harvest at a specified time. this is especially important for agronomic crops and vegetables destined for processing and canning where contracts are signed between the processing company and farmer to deliver a crop on time.
As with all living things, plants are reliant on enzymes to catalyse almost all of their metabolic pathways. the rate of reaction of an enzyme catalysed reaction is highly dependent on temperature- low when cold, increasing with temperatures below the denaturation point. below a certain Base Temperature, enzyme action is so low that the plant 'shuts down' and there is little growth. because of this, we can use the amount of time a plant has experienced above its base temperature to calculate it's growth stage. the method to calculate AHU is as follows:
mean daily temperature = (highest + lowest)/2
daily heat units = mean daily temperature - base temperature
accumulated heat units = sum of daily heat units over period of cultivation
to illustrate this let say we are calculating AHU for pea plants. Yesterday's daily high was 28ºC and its low was 17ºC. Today's daily high was 30ºC and the low was 16ºC. peas have a basal temperature of 20ºC
our mean daily temperatures are as follows:
yesterday: (28+17)/2 = 22.5
today: (30+16)/2 = 23
our Heat units are as follows:
yesterday: 22.5-20 = 2.5
today: 23-20 = 3
our AHU would be the sum of both days, or:
2.5+3 = 5.5
but how do farmers use this? well, each plant requires a certain amount of heat units to complete its growth and for maximum yield. by comparing this with historical data for available heat units in a season, farmers can come forecast the time they need to plant so they will be able to harvest at a specified time. this is especially important for agronomic crops and vegetables destined for processing and canning where contracts are signed between the processing company and farmer to deliver a crop on time.
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