Mulch is one of the simplest, highest-impact upgrades you can make to any garden. Done right, mulching for garden soil locks in moisture, moderates temperature, feeds soil life, suppresses weeds, and sets your plants up for steady growth—through heatwaves, cold snaps, and everything in between. Here’s how to use it like a pro.
What Is Mulch (and Why It Works)
Mulch is any material laid on top of soil. Organic mulches (sugarcane, lucerne, composted bark, wood chip, leaf litter, straw) slowly break down and feed your soil. Inorganic mulches (stones, gravel) protect the surface but don’t add nutrients.
Core mechanisms
- Physical blanket: Reduces evaporation and buffers temperature swings.
- Ecosystem boost: Feeds microbes and beneficial fungi as it breaks down.
- Barrier effect: Blocks light to weeds and shields soil from wind/rain erosion.
Key Benefits of Mulch for Soil & Plants
1. Moisture Retention (Less Watering)
- Cuts evaporation by 25–50% (conditions vary).
- Keeps the topsoil consistently moist so roots grow deeper and stronger.
- Pairs perfectly with drip or soaker hoses under the mulch.
Tip: Aim for 5–7 cm depth (3–5 cm in veg beds). Too thin = little benefit; too thick (>10 cm) can reduce airflow.
2. Temperature Regulation (Happy Roots Year-Round)
- Summer: Insulates against radiant heat; prevents surface “baking.”
- Winter: Cushions cold snaps; roots stay active longer.
- Plants experience fewer stress cycles → steadier growth and flowering.
3. Better Soil Structure & Biology
- Organic mulch breaks down into humus, improving soil tilth and water-holding capacity.
- Encourages worms and beneficial fungi (e.g., mycorrhizae) that help roots access nutrients and water.
- In clay, mulch plus organic inputs gradually loosen compaction; in sand, it builds a “sponge” that holds moisture and nutrients.
4. Nutrient Cycling (Gentle, Ongoing Feeding)
- As mulch decomposes, it trickle-feeds nitrogen, phosphorus, potassium, and trace minerals.
- Reduces nutrient leaching after heavy rain.
- Low, steady nutrition = fewer growth surges and less pest attraction.
5. Weed Suppression & Erosion Control
- Blocks light to seedlings, cutting weed pressure and time spent weeding.
- Shields bare soil from hammering rain and wind, preventing crusting, runoff, and erosion.
6. Cleaner, Healthier Plants
- Reduces soil splash onto leaves (less fungal disease on lower foliage and fruit).
- Keeps fruiting crops (strawberries, cucurbits) off damp soil → fewer rots and blemishes.
Choosing the Right Mulch
For veg gardens:
- Sugarcane, lucerne, pea straw (fine texture, fast breakdown, easy to plant through). Depth 3–5 cm; top up each season.
For ornamentals, shrubs & trees:
- Composted bark, leaf mulch, wood chip (longer-lasting, excellent temperature/moisture control). Depth 5–7 cm.
For natives & low-P lovers:
- Composted bark/wood chip with minimal added phosphorus.
For paths/low-care areas:
- Wood chip or gravel. (Gravel is durable but won’t improve soil.)
Living mulch (groundcovers):
- Native violets, dichondra, thyme, ajuga—great for slopes and continuous cover once established.
Sustainability note: Choose locally produced, certified green-waste mulches where possible. Avoid dyed or contaminated materials.
How to Apply Mulch (Step-by-Step)
- Prep the bed: Remove tough weeds; water deeply so the profile is moist before mulching.
- Add compost (optional but ideal): 2–3 cm of compost under the mulch supercharges soil biology.
- Lay the mulch: Spread evenly to 5–7 cm (3–5 cm for veg).
- Keep a “donut,” not a volcano: Leave a 5–10 cm gap around stems and trunks to prevent rot and pests.
- Irrigation: Place drip/soaker lines under the mulch so water reaches roots directly.
- Top-up cadence: Check depth each season; top up when it drops below ~3 cm.
Common Mistakes (and Easy Fixes)
- Mulch volcanoes: Piled against trunks → rot and pests. Fix: Rake back to create a clear collar.
- Going too thick: >10 cm can restrict gas exchange. Fix: Keep it within the 5–7 cm sweet spot.
- Mixing fresh mulch into soil: Can cause nitrogen tie-up at root level. Fix: Keep mulch on top; let worms do the mixing over time.
- Using hydrophobic soils without prep: Water beads and runs off. Fix: Wet thoroughly first and/or apply a soil wetting agent before mulching.
- Termite risk near structures (AU): Keep woody mulches away from house slabs and inspect routinely.
Special Cases & Pro Tips
- Fresh wood chip “steals nitrogen,” right? On the surface, it’s fine. If you’re worried in veg beds, add a light handful of pelletised organic fertiliser before mulching.
- Clay soils: Mulch + seasonal compost top-dress + occasional gypsum can transform structure over time.
- Sandy soils: Mulch plus biochar (pre-charged) and compost builds long-term water-holding.
- Edible beds: Choose clean, untreated, seed-free mulches; fine textures help with seeding and transplanting.
- Heatwaves: Add an extra centimetre before peak heat to buffer roots; water deeply, then re-cover.
Simple Seasonal Mulch Plan
- Autumn: Top up to 5–7 cm before winter rain/cold.
- Spring: Replenish after pruning/bed prep; integrate drip lines.
- Summer checks: Maintain coverage; patch thin spots (sun-baked gaps waste water).
- Anytime: Keep the stem/trunk collar clear and mulch off house foundations.
When to Call a Pro?
Persistent waterlogging, complex slope erosion, suspected contamination, or tree decline near structures warrants a visit from a horticulturist or soil specialist.
Quick Checklist
- Pre-water beds and apply a wetting agent if needed
- Add 2–3 cm compost under the mulch for maximum benefit
- Spread mulch to 5–7 cm (3–5 cm in veg)
- Keep a 5–10 cm gap around stems/trunks
- Put drip/soaker hoses under the mulch
- Top up seasonally to maintain coverage
Need Mulch?
Diggers Landscape Supplies has the mulch you need — in stock and ready to go. Order now and get the right mix for garden beds, top-dress, turf underlay, and more.
Frequently Asked Questions
How often should I replace mulch?
Top up once or twice a year. Fast-breaking mulches (sugarcane, lucerne) need more frequent top-ups than bark or wood chip.
Can I mulch in winter?
Yes. You’ll still protect soil life, suppress weeds, and buffer temperature swings.
Which is best—straw, bark, or wood chip?
Use strawier, fine mulches for veg/annuals; bark/wood chip for perennials, shrubs, and trees. Match the mulch to the job and how quickly you want it to break down.
Does gravel count as mulch?
It protects the surface and looks tidy, but it doesn’t feed soil. Great for paths and heat-loving xeric plants when combined with organic matter under the gravel layer.
