Most gardens in Heidelberg, Ivanhoe and Rosanna sit on heavy clay. It is sticky in July, cracked in February, and it gets blamed for every plant that dies. Clay is not poor soil. It holds water and nutrients far better than sand does. What kills plants is the way they are put into it.
Do not dig a clay bathtub
The usual mistake is a deep hole filled with bagged potting mix. The soft mix soaks up water, the clay walls will not let it out, and the roots sit in a bucket all winter. By spring the plant is yellow and loose in the ground. Dig the hole wide and shallow instead: two or three times the width of the pot and no deeper than the root ball. Rough up the sides with a fork so they are not glazed smooth. Backfill with the same clay you dug out, broken up, with a little compost mixed through the top.
Plant high
Set the plant so the top of the root ball sits 3 to 5 centimetres above the surrounding soil, then slope the soil up to meet it. That sheds water away from the crown. On a slope, cut a small level shelf and leave a low lip on the downhill side so water soaks in before it runs off.
Gypsum, compost and mulch
Gypsum helps some clays and does nothing for others. Test yours: drop a dry crumb the size of a pea into a glass of rainwater and leave it for a day. If the water turns cloudy, gypsum at about a kilo per square metre will open the clay up over a season or two. If the water stays clear, save your money and add compost. Either way, finish with 50 to 75 millimetres of coarse mulch. It stops the surface baking hard and feeds the worms that do the real digging.
What does well
- Lomandra and dianella: tough in sun or part shade, wet or dry
- Correa: flowers through winter and copes with dry shade under gums
- Poa and kangaroo grass: native grasses for slopes and banks
- Kangaroo paw: only on a raised mound, in full sun
- Roses, salvias and crepe myrtle: cottage plants that like clay
Lavender, rosemary and most grey-leaved plants sulk in wet clay unless the bed is raised. If water is still sitting in a test hole a day after you fill it, that is a drainage problem, and planting technique will not fix it. Have someone look at the fall of the block before you spend money on plants.