How Often to Apply Mulch

Mulch is one of the most valuable tools in a gardener’s arsenal, helping to suppress weeds, retain moisture, regulate soil temperature, and improve soil health. But one question that often comes up is: how often do you need to reapply it?

The answer depends on the type of mulch you’re using, your climate, and how quickly it breaks down.

In this guide, I’ll walk you through how often different types of mulch need refreshing, the signs that it’s time to top up, and proper mulching techniques to get the best results from your garden beds.

How Often Should You Apply Mulch?

Most organic mulches need topping up once or twice a year, while inorganic mulches can last several years with minimal maintenance.

Here’s a breakdown by mulch type:

Organic Mulches (Wood-Based)

• Bark mulch: Every 1–2 years
• Wood chips: Every 1–2 years
• Shredded hardwood: Every 1–2 years

These materials break down slowly and typically need refreshing annually or biannually. If you want to compare different mulch types, wood-based options offer excellent longevity.

Fast-Decomposing Organic Mulches

• Grass clippings: Every 2–4 weeks during growing season
• Straw: Every 3–6 months
• Leaves: Every 6–12 months
• Compost: Every 3–6 months

These nutrient-rich materials decompose quickly, enriching your soil but requiring more frequent reapplication. Grass clippings work well as mulch for vegetable gardens when applied in thin layers.

Inorganic Mulches

• Gravel/stone: Every 3–5 years (mainly to top up settled areas)
• Rubber mulch: Every 5–10 years
• Landscape fabric: Every 3–5 years (replacement)

Inorganic options last much longer but don’t improve soil quality and can be more challenging when dealing with persistent weeds like bindweed.

Signs It’s Time to Reapply Mulch

Rather than following a strict schedule, watch for these indicators:

1. Mulch depth below 5cm (2 inches): The ideal depth is 5–10cm. When it thins below 5cm, it’s time to top up.
2. Visible soil: If you can see bare soil through the mulch, it’s no longer providing adequate coverage, making it easier for dandelions and other weeds to establish.
3. Weeds breaking through: Increased weed growth suggests the mulch layer has become too thin.
4. Faded colour: Fresh mulch has rich colour; faded mulch indicates it’s breaking down.
5. Compacted or matted: Mulch that’s compressed or stuck together isn’t functioning properly.
6. Fungal growth: While some fungi are normal, excessive growth or unpleasant odours may indicate it’s time for fresh mulch, especially if you’re battling ground elder in shaded areas.

Best Times of Year to Apply Mulch

Spring (March–May)

Spring is the most popular time for mulching:

• Apply after soil has warmed up (late April–May in the UK)
• Wait until after spring weeding and fertilising
• Helps retain moisture through the growing season
• Suppresses summer annual weeds

Avoid mulching too early — applying mulch while soil is still cold can delay warming and slow plant growth.

Autumn (September–November)

Autumn is an excellent secondary mulching time:

• Protects roots from winter temperature fluctuations
• Takes advantage of fallen leaves as free mulch material
• Gives mulch time to settle before winter
• Suppresses winter annual weeds

Should You Mulch in Winter or Summer?

• Winter mulching is generally not recommended except for protecting tender plants — frozen mulch can insulate cold soil and delay spring warming.
• Summer mulching can be done if mulch has thinned significantly, but it’s less ideal than spring or autumn application.

How to Refresh Existing Mulch

You don’t always need to remove old mulch completely. Here’s how to refresh it properly:

Step 1: Assess Current Mulch

• Check depth with a ruler or your finger
• Look for decomposition, matting, or contamination
• Remove any diseased or mouldy material

Step 2: Fluff and Rake

• Use a garden fork or rake to loosen compacted mulch
• Break up any matted sections
• This improves air circulation and water penetration

Step 3: Weed Before Topping Up

• Remove any weeds that have emerged
• This is easier to do before adding fresh mulch on top

Step 4: Add Fresh Layer

• Add enough new mulch to reach 5–10cm total depth
• Don’t exceed 10cm — too much can suffocate plants
• Keep mulch 5–7cm away from plant stems and tree trunks

Common Mulching Mistakes to Avoid

1. The Mulch Volcano

Never pile mulch against tree trunks or plant stems. This can cause:

• Rot and disease
• Pest problems
• Girdling roots

Instead, create a donut shape with a gap around the trunk.

2. Over-Mulching

More is not better! Excessive mulch (over 10cm) can:

• Prevent water and oxygen from reaching roots
• Create habitat for pests
• Cause shallow root systems

3. Wrong Mulch for the Job

Not all mulches suit all situations:

• Don’t use fine mulch on slopes (it washes away)
• Avoid fresh wood chips around young plants (nitrogen tie-up)
• Skip acidic mulches around lime-loving plants

4. Mulching Too Early in Spring

Applying mulch while soil is still cold:

• Keeps soil cool and wet
• Delays plant growth
• Can promote slug problems

Wait until soil has warmed to at least 15°C.

How Much Mulch Do You Need?

To calculate how much mulch you’ll need:

1. Measure your garden bed area in square metres
2. Multiply by desired depth (in metres):
   • For 5cm: multiply area by 0.05
   • For 7.5cm: multiply area by 0.075
   • For 10cm: multiply area by 0.10
3. This gives you cubic metres needed

Example: For a 20 square metre bed with 7.5cm mulch depth: 20 x 0.075 = 1.5 cubic metres.

Most mulch is sold in litres (1 cubic metre = 1,000 litres), so you’d need approximately 1,500 litres or about 20–25 standard bags.

Conclusion

Most gardeners should plan to refresh organic mulch once or twice per year — typically in spring and optionally again in autumn. However, your specific schedule depends on:

• Type of mulch (wood-based lasts longer than grass or straw)
• Climate and weather (wet climates speed decomposition)
• Visual indicators (thin coverage, visible soil, increased weeds)

Rather than sticking rigidly to a calendar, observe your mulch throughout the season and top up when depth drops below 5cm or when weeds start breaking through.

By maintaining proper mulch depth year-round, you’ll save time on weeding and watering while creating healthier, more productive garden beds.