Dry weather delays seedling emergence
This year Britain has experienced the driest spring seed beds of any year since 1929 and the second driest in 100 years.
The drier conditions have been caused by the absence of Atlantic weather systems which can usually be relied upon to deliver the bands of rain needed to get seeds to germinate. There was a distinct absence of April showers and May and June remained dry. To make matter worse this abnormally dry spring followed almost without pause from a long cold harsh winter.
What does this mean for sown wildflower and grass seeds?
Complete failures are rare as native wild flowers and grasses are adapted to cope with our changeable weather. Fortunately wild seeds are quite resilient: most are able to sit dormant in the soil for months waiting for the right conditions to arrive. Germination and establishment of seedlings may be delayed, but most sowings will eventually fill in to produce an acceptable result. A bit of patience is usually all that is required.
Read on if you would like more extended advice on what to expect now, how to assess the results of your sowing and whether remedial seeding might be advisable.
What can I expect to see now?
This autumn sowing on this droughty Norfolk soil is more promising than it looks: there are seven species established within this frame.
Autumn sowings of 2009 are producing mixed results. Seed frequently struggled to get established through the winter, a significant proportion of sown seeds remained dormant through the cold weather until spring finally arrived when a second flush of germination occurred. This staggered germination is evidenced now (in July) as populations of young plants: half being well established and near to flowering, the other half seedlings barely 1cm across.
Spring sowings in 2010 are also producing variable results. Seed sown early enough in the spring in settled seedbeds generally found enough moisture to get started. Later sowings into dry or loose seedbeds have been slow to show any signs of growth. In such situations germination will be sporadic and may not fill in until we next get significant rain.
Grass seeds are generally less resilient than wild flowers and decay away more quickly in the soil. As a result what you typically find with spring sowings in dry conditions is that the grass establishment is poor and thin. Amongst this thin open grass the wild flowers germinate sporadically, but finding little competition and having deeper roots are then able to grow unimpeded. This scenario, with the wild flowers having the advantage, can sometimes be quite advantageous towards establishing a good balance of wild flowers.
Of course growth in dry seasons will be slow and any seedlings that have emerged will remain small. Sward structure will as a consequence remain open and ground cover incomplete. For perennial species like the grasses and flowers in meadow mixtures this is not a problem: just a cause of delay until rain arrives to revive them (unless of course the site is subject to extreme drought and everything dies - a rare occurrence on normal soils in the UK).
Weeds on site however can sometimes show a surprising show of growth - how is this?
Sown perennial plants may ‘decide' to sit it out, conserve resources and wait for better conditions rather than risk exposure to drought. Annual weeds by contrast cannot afford this luxury as they need to complete their life cycle and produce seed before the end of the season: they will commit all their available resources to growth and flowering and seeding with a ‘do or die strategy'. Annual weeds are opportunists adapted to exploit temporary bursts of moisture, nutrients or space. A number of seedbeds this spring, for example, have produced a flush of annual sowthistle seedlings after showers which are then able to quickly grow into the space left by the more conservative grasses and perennials. The growth of weeds in dry conditions will not be as vigorous as in a wet climate; however it pays to continue to manage these by mowing as advised, although the number and frequency of cuts will probably be reduced.
Whilst wild flower sowings are quite resilient, the growth and establishment of seeds sown in difficult seasons will not be as complete as that in a good season. Furthermore some sowings this year will have experienced a double hit of both a harsh winter followed by a dry spring. Complete failures are unlikely, but sowings may not have achieved the full results expected.
Assessment of the degree of success or failure is best undertaken in September, preferably after there has been a period of rain to restart growth (assessment of crispy dry vegetation is difficult and misleading as grasses can look superficially quite dead but be capable of revival). Autumn rainfall may also stimulate a fresh flush of germination of sown wild flowers. A September assessment is early enough that it leaves time to consider remedial seeding.
If you are able to find grass plants (however small) every 25-30cm or so, interspersed with 5 -10 or more wild flower plants per square metre, you probably have enough to produce an acceptable result.
Over-seeding in the autumn after establishment is an option where you have a thin open take of either grasses or flowers, or have significant (> 1m2) bare patches. Prepare the site for over-seeding by mowing the established vegetation and any weeds, and remove any arisings. Do not cultivate but simply broadcast seed into the gappy sward.
Either sow the same complete mixture as sown originally, or add seed selectively to make up for observed shortfalls. If, for example, after a long delay to germination the grasses have failed but the wildflowers seem to be coming through, a partial over-seeding of just the grass component of the mixture may be considered (this option is cheaper than the full mix including wildflowers).
This is also a good time to add or boost the yellow rattle component of the sward.
Re-seeding may be the only option if conditions have been so harsh that a complete failure has resulted, or where unforeseen perennial weed problems appear which can only be dealt with by starting again. Before you spray off all growth and start again do make sure that you have properly assessed the results and any avoidable causes of failure or partial failure so that you can avoid simply repeating them the following year. To reseed repeat all stages of site clearance and ground preparation and sow fresh seed.
If you are still unsure what to do with your sowing do contact us for guidance.
Posted on 01 July 2010,