Pea straw vs sugar cane vs lucerne: which mulch?

Side-by-side comparison

The nitrogen difference comes from the plants themselves. Lucerne and field peas are legumes, which fix nitrogen from the air, so their stems and leaves are richer in it than those of a grass like sugar cane or a cereal. Lucerne is cut green as hay, leaves and all, which makes it the richest of the three. Pea straw is what is left after the peas are harvested, so it has less, but still more than cane.

Straw is not hay

The words get used interchangeably at the produce store, but they are different products. Straw is the dry stalks left after a crop has been harvested for its grain or seed, so it should carry few seeds of its own. Hay is a whole crop, usually grass or clover, cut and baled green as animal feed, often with its seed heads. Lucerne is the exception that gardeners want: it is sold as "lucerne hay", but it is cut before it sets much seed. As mulch, straw and lucerne are fine; pasture hay seeds your vegie patch with grass.

How much to buy

Straw is sold by the bale, and bales vary a lot. Compressed sugar cane bales from the hardware store are much smaller than a farm bale of pea straw. So the honest answer is to measure: spread one bale at the depth you want, measure the area it covered, and divide your garden by that number. As a rough guide, published estimates run from about 2–5 m² per small straw bale to about 8 m² for a compressed sugar cane bale, laid 5 cm deep.

A 2.4 m × 1.2 m raised bed (2.9 m²) at 50 mm needs about 145 litres of fluffed mulch, which is one or two bales of most products. A 10 m² vegetable garden at 50 mm needs 500 litres, two to five bales depending on the bale. For bagged mulch and bulk deliveries, the mulch calculator does the arithmetic.

Does straw rob the soil of nitrogen?

Barely, if it stays on top. Soil microbes breaking down carbon-rich material borrow nitrogen to do it. When straw is dug in, that borrowing happens in the root zone and seedlings can go yellow. As a surface mulch the effect is confined to a thin layer where straw meets soil. Lucerne and pea straw have enough nitrogen of their own that it hardly matters. Under sugar cane and cereal straw, give hungry crops a feed of composted manure or pelletised chicken manure before you mulch.

The herbicide residue problem

A small group of weedkillers (clopyralid, aminopyralid and picloram) is used on pastures and some cereal crops, and they are unusually persistent. They survive in the hay and straw, and even pass through animals into manure. In the garden, traces are enough to damage sensitive crops. Tomatoes, potatoes, beans, peas, lettuce and the pumpkin family show cupped, twisted and fern-like new leaves and fail to crop. Home gardeners in Australia have lost whole beds this way.

• Choose legume straw. Lucerne and pea straw are the safer choice, because these herbicides would have killed the crop they came from.
• Ask about cereal straw, hay and manure: where it came from and what was sprayed. Certified organic products are the only guarantee.
• Do the bean test when in doubt. Pot a few broad bean or pea seeds in a mix of the material and potting mix, with a control pot of potting mix alone. Watch the new leaves for three to four weeks. Cupping or twisting in the test pot means leave that material out of the vegie patch.

How to lay straw mulch

1. Water the soil first, deeply. Mulch keeps moisture in, but it also keeps a light shower out of dry soil.
2. Break the bale into biscuits (the slabs it naturally splits into) and tease each one apart. Laying whole biscuits makes a thatch that sheds water.
3. Lay it 50–75 mm deep. Straw settles quickly, so it can look deeper at first. Thicker than about 100 mm and fine straws mat and stay soggy underneath.
4. Keep it 5 cm clear of stems and seedlings. Mulch against stems invites rot, and slugs and slaters shelter in it.
5. Top up every season. Straw mulch rots into the soil, which is the point. Rake the remains in or leave them, and add a fresh layer.

In spring, wait until the soil has warmed before you mulch heavily; a thick layer on cold soil keeps it cold. In bushfire-prone areas, fire authorities advise keeping flammable mulches like straw away from the house walls; use gravel or pebbles right next to the building.

Which one for which job

• Summer vegetable beds: sugar cane or pea straw. Sugar cane lasts through a hot summer and holds moisture well.
• Hungry crops and fruit trees: lucerne. It feeds as it rots and is well worth its price around citrus and heavy feeders.
• Winter and all-round use: pea straw. It is a good balance of feeding and coverage at a middle price.
• Ornamental beds, paths and under trees: none of these. Use a coarse bark or wood-chip mulch that lasts years; see the mulch calculator.

Questions