Technical
Fig 1: Composition of a Single Seed Soil Temperature and Germination
So, with these basic requirements in mind, let’s look at some points for consideration for your pending renovation programmes.
No matter the weather conditions, a lot of grass seed is going to be sown out this coming spring but, at this time of year, it makes sense to focus on soil temperature in relation to germination. Obviously, the goal is to achieve the very best results, whether renovating or establishing new sports surfaces. The first step for the seed in its germination process is the absorption of water. In early spring, there will be plenty of water/moisture available for the seed, but later on, especially if the seed bed preparation is done without regard to the capillary water, water shortage can be fatal.
Germination is strongly related to soil temperature. The higher the soil temperature is in spring the quicker germination will be. Seeds sown in conditions below the basic (minimum) temperature for germination (this value varies from species to species), will not start their germination processes. If a seed needs to wait for suitable
ground temperatures, there is an increased risk of reduced seed viability through either rotting or the food source within the seed becoming depleted as the seed falls in and out of the germination process. Danish research has shown that the
respective grass species require certain accumulated soil temperatures before they will achieve suitable germination
Fig 2: Seed Development: Germination to Establishment
levels. A seeds germination rate can be calculated based on the temperature sum. The temperature sum is defined by the daily temperature the seed is exposed to minus the basic temperature for germination.
The table below shows the basic temperatures that different grass species need in order to initiate the germination
Table 1: Basic Temperatures Poa pratensis
Lolium perenne
Festuca arundinacea Poa annua
3O 2O 4O 2O
C C C C
process. For example Poa pratensis has a basic temperature of 3O
C. Once sown,
the soil temperature will begin to influence the seed germination process. Therefore, one day at 10O
C will add 7O C
to the temperature sum (average temperature minus the basic temperature equals the temperature sum). Furthermore, a weekly average temperature of 11O
C will give a temperature sum of 56O C and so on. The
temperature sum is a vital component in the prediction of the rate of seed germination. Temperature sums can be translated backwards into days. In figure 3, the number of days required for seeds to reach 50% germination can be seen for four different grass species at three
Days from seeding to 50% Germination
different average temperatures. As you can see with all four species,
increased average temperature improves germination rates dramatically. In reality, average temperatures as low as 5O
C are
not ideal to kick start the germination process. These values highlight the importance of temperature as a key factor in this complex process. Only a small increase in daily averages
results in a large change in the temperature sum which, in turn, leads to a very visible reduction in germination time. Equally, higher soil temperatures will result in faster seedling growth and overall establishment. Lolium perenne is, naturally, a rapid germinator, but unfortunately so is Poa annua! Germination test is one of the most important parts of the certification regulations. In the laboratory, Lolium perenne must germinate eighty percent as a minimum, but in lab practice well over ninety is often achieved. The pictures show Lolium perenne seven days after test start, which equals the first counting in Lolium perenne. The shoots are now already approximately seven centimetres high due to optimal conditions (30O
C day temperature for 8 hours + light for 8 hours and 20O C night
temperature for 16 hours), which cannot be reached in the field, even though it is worth a try. The second count, after fourteen days, will normally only add zero or a few percent to the germination. In the field, we also want fast germination in order to maximise germination rates. This, therefore, highlights the significance of the forthcoming months and the weather they may bring. Will there be the prolonged winters of previous years, or will warm south westerly winds bring an early spring? Predicting the climate we will
experience can be left to the weatherman, but judging when best to sow your mixtures lies in your hands in making use of your facilities equipment and local weather conditions. Forcing seed to germinate into conditions which will not sustain good growth thereafter is not good practice!
Fig 3: Temperature Sum Influence on Germination Rate FEBRUARY/MARCH 2012 PC 105
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