Have you ever wondered what makes a weed successful? The list of pointers can be quite long and includes the following:
This list is by no means exhaustive and this article deals solely with germination and dormancy.
Crop plants have been bred to have little or no dormancy so that all the viable seeds will grow immediately post-planting.
However, if all the seeds from an individual weed were to grow in the year following dispersal, the weed could be wiped out in a single season. We know that this does not happen.
Wild plants and weeds have natural dormancy mechanisms which prevent immediate germination in some of the seeds.
This means that some can germinate over a series of years to give the best chance of thriving long-term. Dormancy might be described as a hedging mechanism for weed survival and it is controlled or broken in different ways in different species, as influenced by their natural habitat.
Dormancy can be influenced by things like fire, light access, crop exudates, soil disturbance, available space and conditions for germination.
The breaking of dormancy is complex and understanding this is part of the biology of individual weeds. Dormancy is a natural mechanism to slow or block germination over different timespans. It determines when a seed will germinate and it has evolved to encourage emergence to coincide with the cropping cycle. The range of dormancy observed in just blackgrass seeds alone can vary from 18% to 99% post-harvest.
Dormancy levels are not uniform in every species in every year. We know that dormancy levels will tend to be low, and germination high, when blackgrass flowers in warm, dry conditions, but dormancy will be high and germination low when cold conditions prevail during flowering.
Seeds are subject to two types of dormancy – both of which can be reduced by time. Primary dormancy is given by the mother plant and is active for the first year.
Secondary dormancy cuts in after the first year and holds germination until conditions are suitable.
Dormancy means that some plants will germinate in spring, but research indicates that spring-germinating seeds produce only one-third of the seed numbers per plant, have lower dormancy levels and lower longevity. That is, providing the spring crop itself is highly competitive.
Dormancy has a major effect on seed longevity, with low-dormancy seeds less able to survive over time. However, seed that has naturally high dormancy levels is better suited to long-term survival. So, cultural mechanisms to tackle problem weeds should be influenced by the likely dormancy level set in that year.
In tillage weeds, one of the main triggers appears to be light and/or the sequence of dark and light. This is one of the main reasons why we get a surge of weeds post-cultivation, while relatively few will grow on an undisturbed stubble.
The number of confirmed blackgrass cases in Irish tillage fields continues to spiral, with the grassweed now being found in virtually every tillage county. Regardless, if the weed has already been removed from the field, there is still a chance viable seeds have shed and are waiting to germinate.
Therefore, using the stale seedbed technique is vital to break dormancy of any shed seeds, or indeed seeds buried in the soil seedbank.
Recent research by UK-based agency ADAS has found high levels of dormancy in blackgrass seeds. One test, carried out by researcher Sarah Cook, was designed to break dormancy by growing seed in specific conditions over a fortnight, but only managed to germinate 24% of the samples. These tests were carried out on UK blackgrass populations.
We now know that there is a mixture of UK and native Irish blackgrass populations present in our fields, both of which have different characteristics in terms of herbicide tolerance and possibly even dormancy.
So, growers have to give blackgrass seeds every opportunity they can to germinate in a stale seedbed. This will mean multiple cultivations over a prolonged period.
Blackgrass, like any other grass, needs a fine, firm, moist soil seedbed to germinate. Aim to shallow cultivate stubbles no deeper than 2cm to encourage germination. The aim of this is to encourage good seed-to-soil contact while keeping seeds close to the surface.
Repeat this process multiple times into the autumn, aiming to get a flush of ungerminated seeds after each pass. Spray off the field with a full rate of glyphosate before ploughing/drilling in the autumn. However, remember to delay drilling for as long as possible to give the seeds every opportunity to germinate in advance.