Guideline Index

Chapter 2: Limits to Plant Growth

2.5 Temperature

The amount of heat that reaches the soil is initially influenced by what is held in the atmosphere (clouds, particles, pollution). The amount of heat that is actually retained in the soil is further influenced by ground cover, soil texture and colour.

Soil is quite resilient to marked fluctuations in soil temperature, with this fluctuation decreasing with soil depth. The pasture growth itself will also have a buffering effect on the soil temperature due to its absorption and release of heat. Temperature affects the growth of all pasture plants in many ways; from the weathering of the parent material to the growth rate of soil microorganisms. Generally, how plants respond to temperature relates to the part of the world in which they evolved. Ryegrass has an optimum temperature for growth of around 18°C, while white clover does not reach a maximum growth rate until the temperature has reached 25°C – see Figure 2.2. Paspalum and other tropical C4 grasses, also favour high temperatures. Hence, pastures tend to be ryegrass dominant through the winter, but the level of white clover and tropical C4 grasses often increases greatly in spring and summer.

 

Figure 2.2  Growth rate of ryegrass and white clover in response to temperature
Figure 2.2 Growth rate of ryegrass and white clover in response to temperature

 

Plants require a certain amount of accumulated heat from the sun in order to photosynthesis and produce carbohydrates for growth. Each stage of a plants growth, e.g. from emergence to first mainstem leaf or emergence to full flower, requires a certain accumulated amount of heat units per day to complete that part of its growth. The heat units are accumulated until reaching the threshold temperature (or base temperature) that a plant species requires for normal maintenance. Below the threshold temperature, the plant is in stress or shock and no growth occurs. The number of heat units required at each stage are called “day degrees”. Weather conditions determine whether the day degrees accumulate over a short or longer period. Cool cloudy conditions mean less day degrees, thereby limiting pasture growth as seen from late autumn to early spring.

The formula for calculating day degrees is:

Growing Day Degrees (GDD) =  (Daily max. temp °C – daily min. temp °C) Base Temperature °C
2

The GDD number cannot be a negative; the number for that day would be carried forward as zero. The base temperatures for some common crops and pastures are shown in Table 2.1. Note that some temperate pasture species (e.g. White clover) grow very slowly at a base temperate of 4 – 5°C (Hutchinson et al., 2000).

– Base Temperature °C 2  (Adapted from Fraisse et al., 2012, and Hutchinson et al., 2000)
Base Temperature °C 2 (Adapted from Fraisse et al., 2012, and Hutchinson et al., 2000)