Gel Under Mulch: Does the Mulch Still Earn Its Place?

Every landscape contract that specifies gel eventually gets the same question from the quantity surveyor: if the polymer is holding the water, why are we still paying for 80 mm of bark? It is a fair challenge worth about a third of the planting budget. The short answer is that the two materials block two different losses, and neither does the other’s job. The long answer involves placement, and one mistake that makes the combination worse than either material alone.

Two barriers, two different jobs

Mulch works at the soil surface. A 75–100 mm layer of bark or woodchip cuts evaporation from the top few centimetres by roughly half, keeps the surface cooler through the afternoon, and suppresses weed transpiration — which on a new planting is often the largest single water loss. What mulch does not do is hold water inside the root zone. It slows the exit; it does not create a reservoir.

Gel works 5–20 cm down. It takes water that would have drained below the root zone and holds it where roots can reach it, and it buffers the plant across a dry week. What it cannot do is stop the surface from baking, and a gel-amended soil with a bare, crusted surface still loses most of its water upwards.

Put together, the effect is closer to additive than multiplicative, which is what you would expect from two independent loss pathways. In our own side-by-side blocks on a silt loam, gel at 25 g/m² alone stretched the interval between waterings by about 30 percent, mulch alone by about 35 percent, and the pair by 50–55 percent. That is a real gain, but it is not the doubling some suppliers imply. If your business case needs a doubling, it will not survive a dry season.

The placement error that wastes both

The mistake is spreading dry granules on the surface and then mulching over them. Under a mulch layer, the surface stays humid, so the granules hydrate slowly from vapour and light rain, swell into a continuous slippery mat, and then dry into a hard, water-repellent crust that sheds the next rain. You end up with a sealed layer at exactly the depth where water needs to enter.

Place the gel into the soil, then mulch over the soil. On a bed, incorporate 20–25 g/m² into the top 10–12 cm with a fork or rotavator, water to settle, and then lay the mulch. Around a tree, mix 20–30 g into the backfill, water in with 15 L, and keep the mulch 5 cm clear of the trunk. Never let mulch touch the stem; that is a collar rot invitation unrelated to gel but common on the same sites.

One more placement note: on slopes, apply the gel before the mulch is placed, not after. Granules applied onto a mulch surface wash straight downslope in the first storm and concentrate in the drain — where they do nothing except look alarming.

When to drop one of them

There are sites where mulch alone is the right answer. In a wet temperate climate with 900 mm of rain spread evenly, a new planting rarely experiences the sustained dry-down that gel buffers, and the polymer is an unnecessary cost. Use mulch for weed control and soil temperature and leave it at that.

There are also sites where mulch alone fails and gel alone is not enough: exposed south-facing slopes with shallow soil over rock, where the mulch layer dries through in a heatwave and there is no soil volume to hold a reservoir. There, the combination is justified, and it is also where you should expect the least from both.

Be sceptical of anyone who promises a specific percentage saving without asking about your soil texture, your summer rainfall and your weed pressure. The honest position is that mulch and gel solve different problems, the pair is worth roughly the sum of its parts, and the money is best spent on correct placement rather than on a higher dose. If you need the material numbers to cost the two options against each other, the absorption and dosage specifications give you grams per square metre to work from.