A cover crop is a plant you grow for the soil's sake and then stop. In the northern hemisphere that sentence comes with a calendar: sow in September, sit under snow, kill in April. Australia will not honour it. Our summer is December to February. Our winter is June to August. A large share of our farmland receives its rain in the cool months and then spends four or five months as a dry argument. Another share — northern New South Wales, Queensland, the Top End's farming edges — receives storms in the heat and can grow a jungle in a fortnight, or nothing.

If you plant a cover because an American webinar said "never leave soil bare", and you plant it into a hard, empty profile in January on the Eyre Peninsula, you have not joined a movement. You have sown a bet against the water account.

This is a companion to soil carbon on working farms, because cover crops are often sold as a carbon machine. Sometimes they add a little organic matter. Sometimes they cost you the spring wheat. CSIRO and the Grains Research and Development Corporation have been patient about that distinction.

Learn the Australian year before you buy the seed

Name the seasons as they are used here, not as they are used in a catalogue from Kent.

  • Summer: December, January, February. Heat, fire weather, storm or drought depending on the coast you stand on.
  • Autumn: March, April, May. The break, if there is a break, in the southern cropping zone.
  • Winter: June, July, August. Growing season for wheat, canola, pulses and most southern covers.
  • Spring: September, October, November. The race between grain fill and a cover that has overstayed.

Mediterranean southern Australia — the Western Australian wheatbelt, the South Australian mid-north and Eyre Peninsula, the Victorian Wimmera and Mallee, the southern New South Wales slopes — lives and dies on a cool-season break. Covers that belong there are autumn-sown: oats, barley, rye, vetch, field pea, clover, sometimes radish or a pulse mix. They grow on winter rain. They must be terminated while there is still moisture for the crop that pays the bills.

Summer-rainfall districts — the Liverpool Plains in a decent storm year, the Darling Downs, central Queensland cropping, parts of the northern rivers — can grow a summer cover: lablab, cowpea, millet, forage sorghum, sunn hemp. They can also grow a winter cover on stored moisture if the summer was kind. They cannot grow both at full noise without irrigation or a year that will not come twice.

Tropical horticulture around Mareeba, Bundaberg and the coastal strips has yet another clock, and a weed pressure that will eat a hesitant cover alive. Do not import a Riverina recipe there without talking to the local Department of Agriculture and Fisheries or the relevant state agronomist. The plants will know you are a tourist.

Bare soil is a risk. So is a green soil that has drunk the water you needed in September. Australian agronomy is the art of choosing which risk you can afford.

Households reading this beside a tank should keep the same calendar. A summer cover in a drought-ready Australian garden is often a living mulch that still needs a drink. If the tank is low, a dead mulch may be the more honest cover.

Legumes and cereals are different tools

Cereals — oats, rye, barley, sometimes wheat grown as a cover rather than a crop — are reliable biomass. They hold the ground. They take up leftover nitrogen after a pulse or a fertilised crop, which stops that nitrogen leaving as nitrate in a wet winter on a sloping Chromosol. They make a mulch you can roll or graze. They do not fix nitrogen. They spend it.

Legumes — vetch, field pea, lupin, clover, lablab, cowpea, medic — can add nitrogen if the right rhizobia are present and the soil is not too acid, too dry, or too hostile for nodulation. Australian soils are often exactly those things. Inoculate. Check pH. Do not assume a pretty vetch stand has put on eighty kilograms of nitrogen because a brochure from another continent said so. NSW DPI, Agriculture Victoria and DPIRD publish local figures; they are lower and more useful than the internet.

Mixes exist because the tools are different. A vetch–oat mix on the South West Slopes can give you a carpet and a nitrogen credit. A millet–cowpea mix on the Darling Downs after a winter crop can give you cover through a stormy summer and some N for the sorghum. A radish in the mix can punch a hole in a mild hardpan; it will not shatter a sodic B horizon, and it will not replace gypsum.

Grazing a cover is a legitimate Australian use, especially on mixed farms. It is also how a cover becomes a forage crop that never dies on time. If the cattle are still in it when the profile is drying, you have made a decision about the next crop, whether you wrote it down or not.

Termination is the part that decides the year

The romantic photograph is always of a green stand. The agronomic moment is the kill.

In Mediterranean districts the danger is a cover that grows into September on a drying profile and leaves the wheat or canola to face October with an empty tank. Termination — herbicide, roller-crimper, grazing plus a knock-down, or cultivation if that is still your system — needs to happen while there is still a store. How many weeks before sowing depends on the cover, the soil and the forecast, but the direction of the mistake is always the same: people wait because the stand looks like virtue.

In summer-rainfall country the danger is the opposite delay. A lablab that is still transpiring hard when you wanted to sow wheat into stored moisture is a pump. A millet that has set seed is a volunteer problem. Kill it, keep the mulch, and be honest about the water that left through those leaves.

There is a third failure: terminating so early, or with such aggressive tillage, that you have paid for a cover and kept almost none of its shade. The residue is half the point in a hot season. It lowers the temperature of a black Vertosol surface that would otherwise cook. It slows a storm on a bare rise. It does not, by itself, refill an aquifer.

Irrigation changes the maths and the ethics. A cover grown on Murray–Darling water is a cover grown on a contested account. Householders who want the basin in kitchen-table language can start with the Murray–Darling for householders. On-farm, the question is the same one a water officer would ask: is this biomass worth the megalitre? Sometimes yes, under horticulture, on a shoulder season, with a clear termination date. Sometimes the more regenerative act is to leave the bay dry.

What a cover cannot do in a hot, dry year

It cannot invent rainfall. It cannot turn a Sodosol into a Ferrosol. It cannot sequester a marketed tonne of carbon in a season when the plants themselves are heat-stopped by ten o'clock. It cannot replace the unfashionable work of fixing acidity, sodicity and traffic pans so that the next root has somewhere to go.

What it can do, in the right district and the right window, is keep a surface from blowing, catch a leftover nutrient, add a little nitrogen, and give you a mulch that makes January less violent. Those are real gifts. They are enough. The Land does not need them inflated into a personality.

If you want a practice: map your rainfall first, not your ideology. Read the state agronomy notes for your catchment. Sow a cover only when the profile or the forecast can carry it. Mix a cereal and a legume if you have a reason, not because a packet was pretty. Terminate early enough that the crop you sell still has a spring. And when the year is a dud — when the break fails, when the storms miss the Downs — let the paddock sit with stubble and humility. A cover crop is a tool for a season that has water in it. The hot, dry ones will come anyway. The skill is not covering them with a story.