How to Thicken a Thin Lawn

A thin, sparse lawn is frustrating. You can see soil through the grass, it looks patchy, and weeds have an easy time moving in. The good news is that thickening a lawn isn’t complicated. It just takes the right approach at the right time.

Here’s how to transform a sparse lawn into thick, dense grass that crowds out weeds and looks the part.

Why Lawns Become Thin

Before you fix a thin lawn, it helps to understand why it thinned out in the first place. Otherwise you’ll just end up back where you started.

Common causes of thin lawns:

• Heavy foot traffic compacting the soil
• Shade from trees or buildings
• Poor soil (lacking nutrients or organic matter)
• Mowing too short, weakening the grass
• Drought stress or waterlogging
• Moss or thatch smothering grass
• The lawn was never properly established in the first place

Addressing the underlying cause while you thicken the lawn gives you the best chance of lasting results.

When to Thicken Your Lawn

Timing matters. The best times to overseed and thicken a lawn are:

Spring (mid-March to May): Soil is warming, rain is usually reliable, and grass has all summer to establish. This is the most popular time for lawn renovation.

Autumn (September to mid-October): Actually the ideal time in many ways. Soil is still warm from summer, competition from weeds is lower, and autumn rain helps establishment. The grass establishes roots over winter and comes back strong in spring.

Avoid summer (too hot and dry) and winter (too cold for germination).

The Two Approaches

How you thicken your lawn depends on how bad the problem is:

Overseeding: For lawns that are thin but still have reasonable grass coverage. You’re adding new grass plants to fill the gaps. For a detailed guide to this technique, see how to overseed your lawn.

Full renovation: For lawns that are mostly weeds, moss, or bare soil. Sometimes it’s easier to start again than to rescue what’s there.

This guide focuses on overseeding, the approach that works for most thin lawns.

How to Thicken a Thin Lawn: Step by Step

Step 1: Mow short

Cut the existing grass shorter than usual, around 2-2.5cm. This reduces competition for the new seed and lets light reach the soil surface. Collect the clippings.

Step 2: Scarify or rake vigorously

Scarifying removes thatch (the layer of dead material at the base of the grass) and opens up the soil surface. This is essential. Seed sitting on top of thatch won’t germinate properly.

For light work, a spring-tine rake pulled firmly through the grass works. For larger areas or heavy thatch, hire a powered scarifier. The lawn will look terrible afterwards. That’s normal and means you’ve done it properly.

Step 3: Aerate if needed

If the soil is compacted (hard to push a screwdriver in, water pools on the surface), aerate before overseeding. This helps roots establish in the soil rather than sitting on a hard surface.

For overseeding, spiking with a garden fork or rolling aerator is usually sufficient. Hollow-tine aeration is even better if you have access to one.

Step 4: Apply pre-seed fertiliser

New grass seedlings need phosphorus for root development. Apply Pre-Seed Foundation Feed before sowing. Its high phosphorus formula gives seedlings the nutrients they need right from the start, without the herbicide that would harm new growth.

Don’t use a high-nitrogen feed at this stage. It encourages the existing grass to grow vigorously and out-compete the new seedlings.

Step 5: Overseed

Spread grass seed evenly over the lawn. For overseeding, use around 25-35g per square metre (roughly half the rate you’d use for a new lawn).

Choose a seed mix that matches your existing grass and suits your conditions. For spring renovation, Spring Lawn Seed is optimised for spring soil temperatures. For shady areas, use a shade-tolerant mix. For heavy traffic, choose a hardwearing mix. Not sure which seed is right? See our Best Grass Seed UK buyer guide.

A handheld spreader helps achieve even coverage. For small areas, you can broadcast by hand. Just divide the seed in half and apply in two passes at right angles to each other.

Step 6: Topdress (optional but recommended)

Topdressing means spreading a thin layer (5-10mm) of soil, compost, or sand over the lawn. This:

• Covers the seed, improving germination rates
• Helps level minor bumps and hollows
• Adds organic matter to improve soil structure
• Protects seed from birds

Use a sandy loam or purpose-made lawn topdressing. Spread it thinly and work it into the grass with a rake or the back of a rake. You shouldn’t bury the existing grass completely.

Step 7: Water

Keep the seedbed consistently moist for the first 2-3 weeks. This usually means watering lightly every day or two in dry weather. The seed must not dry out once germination has started.

Once the new grass is established (after 3-4 weeks), gradually reduce watering frequency but water more deeply when you do.

Step 8: First mow

Wait until the new grass is about 5-6cm tall before the first cut. Set the mower high (4cm) and make sure the blade is sharp. New grass pulls out easily if the blade tugs rather than cuts cleanly.

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

How Long Until It Thickens?

Grass seed typically germinates in 7-21 days depending on the variety and conditions. You’ll see the lawn starting to fill in within 4-6 weeks.

However, a properly thick lawn takes a full growing season to develop. The new plants need time to tiller (produce multiple shoots from each plant) and spread. Be patient. By the following spring, you should have a noticeably denser lawn.

Maintaining Thickness Long-Term

Once you’ve thickened your lawn, keep it that way:

Feed regularly: Seasonal feeding keeps grass healthy and vigorous, able to fill gaps naturally and resist weeds. Once your new seed has established (6-8 weeks after germination), apply a seasonal lawn treatment to nourish the whole lawn and prevent weeds from competing with new growth.

Mow at the right height: Cutting too short weakens grass and opens up space for weeds and moss. Keep it at 3-4cm for most lawns.

Address problems early: Deal with moss, compaction, or drainage issues before they thin the lawn out again.

Overseed annually: A light overseed every autumn keeps grass density topped up and introduces new, vigorous plants.

For more lawn improvement guides, browse our full lawn care resource.

Frequently Asked Questions

Can I just throw grass seed on a thin lawn?

You can, but results will be poor. Seed needs soil contact to germinate properly. At minimum, rake the lawn first to open up the surface. Better results come from the full process: scarify, seed, topdress, water.

How much grass seed do I need?

For overseeding, use 25-35g per square metre. A typical small lawn (50 square metres) needs about 1.5-2kg of seed.

Should I use the same grass seed as my existing lawn?

Ideally, yes, or at least something similar. Using a very different variety can create a patchy appearance with different colours and textures. If you’re not sure what you have, a general-purpose mix usually blends in acceptably.

Will overseeding get rid of weeds?

Indirectly. Thick, healthy grass crowds out weeds by competing for light, water, and nutrients. But overseeding won’t kill existing weeds. Deal with those separately before or after the new grass establishes.

Can I walk on the lawn after overseeding?

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

What if the thin areas are in shade?

Use a shade-tolerant grass seed mix and accept that grass in heavy shade will always be sparser than grass in sun. You may also need to improve light levels by pruning overhanging branches.