Grass Seed Calculator

Lawn Seeding Guide

What should I know about Seeding Rates?

Standard UK grass seed application rates: new lawn from seed 35-50g per m². Overseeding thin areas: 25-35g per m². Full renovation (after scarification): 35-50g per m². These rates are higher than many packet instructions because UK conditions (rain, slugs, birds) reduce germination success rate compared to ideal laboratory conditions. Always apply at the higher end of the recommended range for new lawns — thin germination is the most common new lawn problem and adding more seed is much cheaper than dealing with a patchy, weed-prone lawn later.

What should I know about Soil Preparation?

Soil preparation is the most important factor in successful lawn establishment. New lawns: remove all weeds (glyphosate-based weedkiller, then 2 weeks wait), rotovate to 15-20cm, remove stones and debris, firm with a roller or trampling (aiming for a fine, level tilth), rake to a fine surface. Overseeding: mow closely (3-4cm), scarify or rake vigorously to create soil contact, apply seed, top dress with a thin layer (5-10mm) of lawn dressing or compost to improve seed-soil contact. Seed contact with soil is the single biggest factor in germination success — seed left sitting on top of undisturbed ground or thatch will dry out and fail to establish.

What should I know about Timing and Germination?

Best sowing times in the UK: mid-August to mid-October (preferred) — soil is still warm from summer, autumn rains provide moisture without the need for constant irrigation, and competition from weeds is lower. March to May (second option) — soil warming from winter, but spring weeds compete. Avoid sowing in summer (drought risk without irrigation) or December to February (cold soil prevents germination). Germination time: 7-14 days in ideal conditions (15-25°C soil temperature, consistent moisture — allowing the seedbed to dry out during this window is one of the most common reasons new grass seed fails to establish).

What should I know about Lawn Care After Seeding?

Water daily in dry conditions until germination, then every 2-3 days as the lawn establishes (about 6 weeks). Do not let the seed dry out during germination — a 10-minute dry period during the germination stage can kill emerging seedlings. First feed: apply a pre-seeding fertiliser before sowing (high phosphorus to encourage root development) or a spring lawn feed 4-6 weeks after germination. Birds can be a significant problem — use a flutter tape or decoy predator over the seeded area for the first couple of weeks until the grass is established enough that birds pulling at emerging shoots is less of a concern.

What should I know about Seeding Rate and Why More Isn't Better?

Standard UK lawn seeding rates are around 25-35g/m² for a new lawn and 15-20g/m² for overseeding/renovating an existing thin lawn — significantly less than many first-time seeders instinctively apply. Sowing too heavily doesn't produce a thicker lawn faster; instead, overcrowded seedlings compete intensely for light, water, and nutrients, often resulting in weak, thin grass more prone to disease (particularly damping-off fungal disease in dense, moist seedling clusters) than a correctly-rated sowing would produce. Species mix also affects the ideal rate: fine-leaved ornamental mixes (bent and fescue grasses) are typically sown at the lower end of the range since the seeds are smaller and germinate more densely per gram, while coarser utility/family lawn mixes (ryegrass-heavy) often sit at the higher end since ryegrass seed is larger and establishes less densely per gram sown.

What do I need to know about Soil Temperature and Sowing Timing?

Grass seed needs soil temperature consistently above about 8-10°C for reliable germination, which in most of the UK means the two reliable sowing windows are spring (April-May) and early autumn (September-early October) — soil is both warm enough and reliably moist in these windows, while mid-summer sowing risks the seed drying out faster than it can establish, and late-autumn/winter sowing risks the seed simply sitting dormant (or rotting in waterlogged, cold soil) until spring. Autumn sowing is often preferred by experienced gardeners over spring because weed competition is typically lower in autumn and the grass has a full growing season ahead before the following summer's heat stress, whereas spring-sown grass faces weed competition and summer drought stress while still young and establishing its root system.

What should I know about Choosing a Grass Seed Mix for Your Lawn Type?

Grass seed mixes are blended for different purposes, and choosing the wrong one is a common reason a new lawn underperforms regardless of how carefully it was sown. Hard-wearing or 'family' mixes contain a high proportion of ryegrass, which germinates fast and tolerates heavy foot traffic but has a coarser leaf and less refined appearance. Fine or 'ornamental' mixes use bents and fescues for a dense, soft, close-mown lawn, but these are slower to establish and less tolerant of wear, making them unsuitable for a garden used daily by children or pets. Shade mixes contain fescues bred to cope with reduced light under trees or near fences, where standard mixes thin out badly. Low-maintenance or wildflower-grass mixes are suited to areas mown less often. Always check whether a mix is intended for overseeding an existing lawn (often finer-leaved, to blend in) or starting from bare soil, since some overseeding mixes establish quickly in gaps but aren't meant to form a lawn from scratch on their own.

Choosing a Seed Mix by Lawn Type and Use

Grass seed mixes are blended for different purposes, and choosing the wrong one for how a lawn is actually used is a common reason new lawns struggle even when sowing rate and timing are done correctly. (For specifically repairing bare patches or overseeding an existing lawn rather than sowing a new one from scratch, see this site's lawn seed calculator, which covers patch-repair rates and differentiated overseeding vs new-lawn rates in more detail.)

Ryegrass-dominant mixes establish fastest and tolerate the most foot traffic, making them the standard choice for family gardens, but they're also more prone to invasive-looking coarse texture if mowed too high. Fine ornamental mixes give the soft, even "bowling green" look many people want for a feature lawn, but they establish more slowly and tolerate far less wear — choosing this mix for a lawn that will actually host regular football games or a trampoline area typically results in disappointment within the first season.

Worked Example: Sowing a New 80m² Lawn from Bare Soil

A new-build homeowner has 80m² of bare, levelled topsoil in the back garden and wants a hard-wearing family lawn suitable for children and a dog. This calls for a ryegrass-dominant mix at a new-lawn rate of around 35g/m². Seed needed: 80m² × 35g/m² = 2,800g, or 2.8kg. Seed is commonly sold in 1kg (covers roughly 25-30m²), 5kg, or 10kg bags — an 80m² lawn needing 2.8kg would typically be bought as a single 5kg bag, since buying three separate 1kg bags is usually more expensive per kilogram and risks running slightly short if application isn't perfectly even. Applying the seed in two passes at half the rate each, walking in a different direction for the second pass, is the standard technique for achieving even coverage — splitting 2,800g across two passes of 1,400g each, roughly 17.5g/m² per pass. After sowing, raking lightly to cover most seed with 3-5mm of soil, then keeping the area consistently moist for 10-21 days (ryegrass typically germinates fastest, visible within 7-14 days under good conditions), is what turns a correctly calculated seed quantity into an actual established lawn — seed quantity gets the maths right, but watering discipline in the first three weeks determines whether that maths translates into a full, even lawn or a patchy one needing a second overseed.

Germination Time by Grass Species and Soil Temperature

Different grass species germinate at noticeably different speeds, and soil temperature — not air temperature — is the deciding factor for when seed will actually sprout.

Soil temperature typically lags a week or two behind air temperature in spring, so sowing based on a warm-feeling day rather than a measured soil reading is a common cause of patchy or failed germination — a basic soil thermometer pushed a few centimetres into the ground gives a far more reliable sowing signal than the forecast alone.