How to Overseed Your Lawn

Overseeding is one of the most effective ways to improve a tired lawn. By introducing fresh grass seed into an existing lawn, you fill thin areas, increase density, and introduce new, vigorous grass plants that help crowd out weeds and moss.

Here’s how to overseed your lawn properly – and why it’s worth doing every year.

What Is Overseeding?

Overseeding simply means spreading grass seed over an existing lawn rather than starting from bare soil. The new seed germinates and grows alongside your existing grass, filling gaps and thickening the overall lawn.

It’s different from patching bare areas (which targets specific spots) or full renovation (which involves killing the existing lawn and starting again). Overseeding works with what you have to make it better. For a comprehensive approach to lawn improvement, see our guide on thickening a thin lawn.

Why Overseed?

Grass plants don’t live forever. Individual grass plants typically live 5-7 years before declining. Without new plants coming through, your lawn gradually thins over time.

Benefits of regular overseeding:

– Fills thin areas before weeds colonise them
– Increases lawn density for better weed resistance
– Introduces newer grass varieties with improved disease resistance
– Repairs wear and tear from foot traffic, pets, and weather
– Improves overall colour and appearance
– Maintains lawn quality without expensive renovation

Professional groundskeepers overseed annually as standard practice. It’s preventative maintenance that keeps a lawn in top condition.

When to Overseed

Timing is crucial for successful overseeding. Grass seed needs warmth and moisture to germinate, and the new seedlings need time to establish before challenging conditions arrive.

Autumn (September to mid-October) – Best time

Autumn is the ideal time to overseed. The soil is still warm from summer, air temperatures are cooling (reducing stress on new seedlings), and autumn rain provides natural moisture. New grass has time to establish roots before winter and comes back strong in spring.

Use Autumn Lawn Seed which is formulated for these conditions.

Spring (mid-March to May) – Second best

Spring overseeding works well but faces more competition from weeds (which also germinate in spring). The grass has the whole growing season to establish, which is an advantage.

Use Spring Lawn Seed which germinates reliably in cooler spring soil temperatures.

Avoid summer and winter

Summer is too hot and dry – seed either fails to germinate or new seedlings die from drought stress. Winter is too cold for germination. Stick to spring or autumn for the best results.

How to Overseed: Step by Step

Step 1: Mow short

Cut your lawn shorter than usual – around 2-2.5cm. This reduces competition from existing grass and allows light to reach the new seed. Collect the clippings rather than mulching.

Step 2: Scarify

Scarifying is essential for successful overseeding. It removes thatch (the layer of dead material at the base of the grass) and opens up the soil surface so seed can make contact with the soil.

Seed sitting on top of thatch won’t germinate properly – it needs soil contact. Use a powered scarifier for larger lawns or a spring-tine rake for small areas. The lawn will look terrible afterwards – that’s normal and a sign you’ve done it properly.

Step 3: Aerate (optional but recommended)

If your soil is compacted, aeration before overseeding dramatically improves results. The holes allow seed to settle into the soil and provide channels for roots to establish.

Hollow-tine aeration is ideal if you can access one. Spike aeration with a garden fork or rolling aerator also helps.

Step 4: Apply pre-seed fertiliser

Spread pre-seed fertiliser over the lawn. This provides phosphorus for root development – crucial for new seedlings. Unlike regular lawn feed (which contains weedkiller that would prevent seed germinating), pre-seed fertiliser is seed-safe.

Step 5: Spread the seed

Spread grass seed evenly over the entire lawn. For overseeding, use approximately 25-35g per square metre – roughly half the rate you’d use for a new lawn from scratch.

For even coverage:

– Divide your seed in half
– Apply the first half walking in one direction
– Apply the second half walking at right angles to the first
– This cross-pattern ensures you don’t miss any areas

A handheld broadcast spreader makes this easier for larger lawns. For small lawns, hand broadcasting works fine.

Step 6: Work the seed in

Lightly rake the seed into the soil surface or brush over the lawn with a stiff broom. You’re not trying to bury the seed deeply – just ensure good soil contact. Some gardeners apply a light topdressing of compost or topsoil at this stage, which helps cover the seed and improves germination rates.

Step 7: Water

Water the lawn thoroughly after seeding. Then keep the seedbed consistently moist for 2-3 weeks until the new grass is established. This typically means watering lightly every day or two in dry weather.

The critical rule: once seed starts germinating, it must not dry out or the seedlings will die.

Aftercare

First mow

Wait until the new grass is at least 5cm tall before mowing. Set the mower to its highest setting (around 4cm) and ensure the blade is sharp. New grass pulls out easily, so a clean cut is essential.

Gradually reduce the mowing height over subsequent cuts until you reach your normal height.

Reduce traffic

Minimise foot traffic on the lawn for 4-6 weeks while the new grass establishes roots. Light walking is fine after 3-4 weeks, but avoid heavy use or playing on the lawn until the new grass is well rooted.

Resume normal feeding

Once the new grass has been mowed 3-4 times, you can return to your normal feeding schedule. If you overseeded in autumn, the next feed would typically be your winter treatment. If you overseeded in spring, continue with summer feeding.

How Much Seed Do I Need?

For overseeding, use 25-35g per square metre. Here’s a quick guide:

– 25m² lawn (5m x 5m): 625g-875g (roughly 1 bag)
– 50m² lawn: 1.25-1.75kg
– 100m² lawn: 2.5-3.5kg
– 200m² lawn: 5-7kg

If your lawn is particularly thin or worn, use the higher rate. For maintenance overseeding of an already decent lawn, the lower rate is sufficient.

Choosing the Right Seed

Match your seed to your lawn’s conditions and your existing grass:

Autumn Lawn Seed: Formulated for autumn conditions – warm soil, cool air. Establishes strong roots before winter.

Spring Lawn Seed: Germinates reliably in cooler spring soil temperatures. Gets your lawn summer-ready fast.

Fast Growing Grass Seed: Germinates in 7-14 days. Good for quick results or if you’re overseeding later in the season.

Shade areas: Use Shade Tolerant Grass Seed for areas under trees or buildings.

Frequently Asked Questions

How long until I see results?

Grass seed typically germinates in 7-21 days depending on variety and conditions. You’ll see the lawn thickening noticeably within 4-6 weeks. Full results take a growing season as the new plants mature and spread.

Can I overseed without scarifying?

You can, but results will be much poorer. Seed needs soil contact to germinate. At minimum, rake the lawn vigorously to open up the surface. Proper scarifying dramatically improves germination rates.

Should I overseed every year?

Annual overseeding is ideal – it’s what professionals do. At minimum, overseed every 2-3 years to maintain lawn density. If your lawn is already thick and healthy, you can skip years, but thin or worn lawns benefit from annual attention.

Can I use weedkiller before overseeding?

Most lawn weedkillers leave residue in the soil that prevents grass seed germinating for 4-6 weeks. If you need to treat weeds, do so at least 6 weeks before overseeding, or wait until spring and overseed first (then treat weeds once the new grass is established).

What if it doesn’t rain after overseeding?

You’ll need to water. Keep the seedbed moist (not waterlogged) for 2-3 weeks. In dry autumn weather, this might mean watering daily. The seed must not dry out once germination has started.

For more seeding guides and lawn care advice, explore our lawn care article hub.

Ready to thicken your lawn? Choose Autumn Lawn Seed for September-October overseeding or Spring Lawn Seed for March-May. Pair with Pre-Seed Foundation Feed for the best establishment.