Patchy lawn? Find the cause before you buy seed

The standard response to a patchy lawn is a $30 box of "miracle" patch mix from the hardware shop, scattered with hope. Three weeks later: same patches, fatter pigeons. Seed comes last. First, work out why that spot is bare - something made it bare, and it's almost always still there.

Diagnose first

• Patches along the path, the clothesline run, the cricket pitch. Traffic and compaction. The soil's been trodden into concrete and roots can't get air or water into it. If a screwdriver won't push in easily, this is you.
• Thinning under trees or on the south side of the house. Shade. Most lawn grasses want 3-4 hours of direct sun; couch wants more. No seed survives what killed its predecessor.
• Patches that lift like carpet, magpies drilling the lawn. Curl grubs have eaten the roots. Treat the grubs before repairing or you're laying out a second dinner.
• Round spots where the dog goes. Urine burn - see the dead grass guide for the tell-tale green ring.
• Scalped crowns and high spots. The mower's shaving the lawn's high points down to dirt. The fix is cutting height, or levelling the lumps - new grass will just get scalped too.
• General moth-eaten thinning everywhere. Usually hunger plus mowing too short. A lawn cut to the floor every fortnight and never fed will thin out; weeds fill the gaps.

The repair, in order

1. Fix the cause. Aerate the compacted run (a garden fork worked back and forth every 10 cm does a real job on small areas). Treat grubs. Raise the mowing height. Redirect the dog. Accept that deep shade might want pavers or shade-tolerant plants instead of a fifth attempt at grass.
2. Rough up the surface. Scratch the patch back to loose soil with a rake - seed and runners need soil contact, not a thatch mat to sit on.
3. Repair to match your grass. The bit the seed box doesn't mention: if your lawn is couch, kikuyu or buffalo - and in Melbourne's north it usually is - it doesn't grow from the seed in that box anyway. Those lawns repair from runners: feed and water the surrounding lawn and it'll walk back across a small patch in a warm month or two, or cut a strip of turf for instant coverage. Seed is for fescue and rye lawns, sown thick in early autumn or mid-spring.
4. Water like it's a nursery. Light and daily until establishment, which is the one time shallow watering is correct - then taper to deep and infrequent.
5. Keep traffic off. Four to six weeks. Yes, including the dog. Good luck.

When it works

Warm-season repairs take between October and December - the grass has the whole summer to knit in. Fescue goes in around March-April while the soil's still warm but the heat's gone. Mid-winter repairs fail, and February repairs need daily babysitting through the heat. Time it right and a patchy lawn is a one-season fix; keep it fed and mown a notch higher, and the patches don't come back.