Getting the numbers right

I’ve been wanting to establish a front border of lilly pilly shrubs for a while.

It’s a twenty metre stretch, so that means twenty one plants.

I went with Syzygium ‘Cascade’ as they stay below two metres tall, have lovely fluffy pink flowers followed by bright red berries, and flushes of pink drooping new leaves.

All in all, a great choice for an informal attractive native screening hedge.

Our front has pretty poor soil, so I knew I’d need to buy in some better soil to give the shrubs a chance.

The plan was to dig oversized holes and fill these with a soil mix … but how much soil would I need to buy in, especially when most landscaping companies sell soils, mulches and gravels by the cubic metre?

The easiest way is to draw a sketch of what you want to do, make some measurements of the length and width, and take that to one of our landscaping suppliers and they’ll be more than happy to work everything out for you.

If you’d like to do it yourself, here’s a simple refresher to help in your calculations.

Working out volumes of mulch and soil becomes very easy using these formulas.

Area of a square (and rectangle): l x w (l = length; w = width)

Circumference of a circle: π x d (π = pi, which = 3.14; d = diameter)

Area of a circle: π x r2 (r = radius = half the diameter)

Area of a triangle: (b x h)/2 (b = base; h = height)

These formulas only give you the area, the other variable in working out volume is depth. And depth depends on what you need. And remember that your average garden soil mix will compact down a little with time, so allow an extra 15% to compensate.

So to work out how much soil to order for garden beds, work out the area of all the beds. You then multiply the area by the depth, plus 15%. Make sure you use all the same units of measurement – it’s best to convert everything to metres – and be careful with decimal points.

10mm = 1cm = 0.01m

Mulch is best laid at about a 70mm thickness, so you multiply the garden bed area by 70mm – again make sure you keep the units the same. Don’t multiply centimetres by millimetres.

And a formula that is very useful when looking to fill a pot plant (or a hole) is the volume of a cylinder, as you will be able to work out how much potting mix to purchase.

Volume of a cylinder: π x r2 x h

It’s also handy to know how much a cubic metre weighs – especially if you’re taking it away yourself in a trailer or in the back of a ute.

1 cubic metre

top soil = 1.4 tonnes

garden soil = 1.2 tonnes

wood chip = 0.4 tonnes

bark = 0.3 tonnes

leaf/chip forest blend = 0.3 tonnes

brickies sand = 1.45 tonnes

coarse river sand = 1.5 tonnes

gravel = 1.5 tonnes

deco-granite = 1.3 tonnes

30L bag of potting mix = 15kg

So in my case, I dug a hole 50cm or 0.5m across and 0.5m deep. Using the volume of a cylinder formula (π x r2 x h) each hole would be 3.14 x (0.25 x 0.25) x 0.5 = 0.098, with the answer being in cubic metres.

Twenty one holes then gives me 2.058 cubic metres of soil that I needed. Allowing for compaction, I ordered 2.25 cubic metres which did the job perfectly.