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When to sow grass seed?

You want to establish a lawn from seed, you have bought your grass seed and you are ready to sow. But, when is the right time to sow grass seed to establish a lawn from scratch? The key to establishing a brilliant lawn from seed is to find the right time of the year to sow. In this article, we will discuss when is the best month to sow grass seed in the United Kingdom and other important factors to consider before you start sowing your lawn.

What month is best for sowing grass seed?

The best month to sow grass seed in the United Kingdom is generally from mid – Spring through to late-Autumn when soil temperatures are warming up and air temperatures are climbing. Sowing later in Spring or early summer can also be successful but it is likely that sowing too late autumn/winter sowing will result in poor germination rates due to cold soil temperatures and wetter weather conditions.

Sowing grass seed in the spring can sometimes result in more annual and perennial weeds. There are a few reasons for this increase in competition from weeds:

  1. Weed Competition: Spring is a time when many weed seeds also germinate and start growing vigorously. When you sow grass seed during this season, your newly germinated grass may have to compete with these fast-growing weeds for sunlight, water, and nutrients. Weeds are often more aggressive in spring, potentially outcompeting the grass seedlings.
  2. Cool Soil Temperatures: Weeds often begin to grow in lower soil temperatures than grass. The result is that soil conditions favour the weed species over the grass. As the temperature increases lawn weeds may establish themselves more quickly in the warmer soil, posing a challenge to the newly sown grass. This is often the case in clay soils that are slow to warm up in the spring, as opposed to sandy soils that will war up quicker. Shady lawns are often cooler in the spring and may be slower to germinate.
  3. Inconsistent Weather Conditions: Spring weather can be unpredictable, with fluctuating temperatures and varying levels of rainfall. These conditions may create stress for the newly planted grass, making it more susceptible to weed invasion. Weeds are often hardier and can adapt to changing conditions more effectively than newly germinated grass seedlings.
How to minimise weed seed germination:
  • Preparation: Prepare the soil thoroughly before seeding to reduce the existing weed population.
  • Weed Control: Treat the area with a herbicide, such as glyphosate, that will kill any existing plants but will not affect the new sowings. However, be cautious with herbicide use, as some products may also inhibit the germination of grass seeds.
  • Monitor and Remove Weeds: Regularly inspect the area for emerging weeds and remove them promptly to reduce competition with the grass seedlings.
  • Choose the Right Grass Species: Select grass varieties that are well-suited to your specific climate and growing conditions. Some grass species may be more resilient against weed competition because they are faster to establish, such as seed mixes containing perennial ryegrass.

By taking these precautions and being attentive to weed management, you can increase the chances of successful grass establishment even when sowing in the spring.

Sowing seed in autumn sowing is very effective due to the combination of high soil temperatures and high air temperatures. To establish your lawn from seed keeping constant moisture around the seed for a couple of weeks is very important. A light watering to keep the soil moist will always help, but in the autumn there is usually more frequent rainfall so this becomes less of a necessity. Another benefit of autumn sowing is that you will not have the danger of heading into a period of hot, dry weather. If you do not have water available, sowing in the spring can be risky if conditions turn dry.

The Royal Horticultural Society has a guide on controlling weeds.

What is the latest time of year to plant lawn seed?

In the United Kingdom, a relatively limited set of grass species is commonly utilized. Perennial ryegrass, red fescues, and bents collectively make up more than 90% of the grass seed employed for lawns in the UK. However, these three species exhibit distinct requirements regarding germination temperatures, influencing the optimal timing for sowing. Here are the temperatures at which one can anticipate favourable germination:

  • Perennial ryegrass: 7°C (with a germination period of 7-14 days)
  • Red fescues: 11°C (with a germination period of 11-21 days)
  • Bent: 15°C (with a germination period of 11-21 days)

While germination can occur below these temperatures, the process is likely to be prolonged due to the biochemical processes of germination slowing down under cooler conditions.

It’s evident that soil temperatures tend to rise in mid-spring and summer, gradually decreasing as we move into autumn and winter. However, the ground exhibits a slow response to warming up and cooling down. This means that, during spring, while air temperatures may be favourable, soil temperatures can lag behind for a considerable period. This characteristic poses challenges, particularly if dry spells occur as we approach summer. In our current climate, achieving optimal conditions can be tricky, with dry spring conditions being a common concern. Despite the warmth in April, the soil may retain its coolness. It’s likely safe to sow, but the germination process might take a bit longer than anticipated.

Grass Seed Germination Temperatures

Grass Seed Germination Temperatures

The provided chart illustrates soil temperatures in Edinburgh throughout a 12-month span, juxtaposed with the temperatures required for optimal germination of the three primary grass seed species. The observations reveal significant distinctions.

  • Perennial ryegrass: Recommended sowing period extends from mid-March to mid-November.
  • Red fescues: Optimal sowing spans from mid-April to late October.
  • Bent: Ideal sowing time is from June to mid-September.

Conclusion: A standout finding is that perennial ryegrass boasts the broadest sowing window, with late summer being the prime period. Seeding in autumn offers the best chance for successful lawn establishment. However, if early spring or late autumn sowing is preferred, incorporating perennial ryegrass in the mixture is advisable. Even at less-than-ideal temperatures, grass species can still germinate, albeit at a slower pace. Grass seeds can endure extended periods in the soil before germinating, but prolonged wetness may lead to rot. Choosing when to plant grass seed in the UK is crucial, and when in doubt, opting for a mix with ryegrass is a wise choice.

Additional considerations:
  • Seeding in autumn reduces weed competition, especially from annual plants like thistles, whose germination aligns with spring.
  • Soil type matters; moist clay soils may take longer to warm up in spring, while sandy soils generally offer warmer conditions, conducive to grass seed germination.
  • For winter seeding, grass seed mixtures containing annual ryegrass exhibit better germination in cold soil temperatures.
  • Adequate moisture is crucial during seeding, but heavy rain should be avoided, as it may wash away seeds or create a soil surface cap, hindering germination.