Hi @spiltmilk,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us, and this looks like an exciting project for you to get stuck into.
Being a fairly steep slope, the biggest risk is erosion, and mulch alone will always slide over time unless something is holding it in place. Adding some form of stabilisation across the slope will make a huge difference. This can be done with low timber retaining structures using garden edging or timber sleepers, or even pinned jute or coir matting if you want a softer, more natural look. These barriers slow water flow and stop mulch from creeping downhill. Sha
You'll then want to lay down some weed mat. Choose a heavy-duty weed mat such as Strol Landscape Pro 120 Weedmat. Lay it horizontally across the slope, overlap joins generously, and pin it down well with weed mat pins every few hundred millimetres. Cut neat crosses where plants will go and tuck the fabric back under itself, so it stays secure.
Mulch choice is also critical. Lightweight mulches wash away quickly, especially in heavy rain. Pine bark or hardwood mulch works best because they interlock and have enough weight to stay put. You'll want to apply a thinner layer than you would on flat ground, around 50 to 70 mm, so it does not act like a sliding blanket.
Plants are what really lock the slope together over time. Agave works well for structure, but combining it with tough ground-holding plants will dramatically reduce maintenance. Clumping grasses, low shrubs, and spreading groundcovers will help to bind the soil and shade out weeds. Plant in a staggered pattern across the slope rather than straight rows so roots knit together more effectively.
With these steps combined, you will be able to create a wonderful low-maintenance garden on this sloping area.
Let me know if you have any further questions.
Jacob
