Cover-crop trials in Paarl

The results so far

“Cover-crop cultivation is a long-term process,” says Howell. “All the research has shown this.” Nonetheless, her data from the first two seasons (2023 and 2024) already display some promising trends.

In 2023, cover crops and weeds produced ample dry matter, despite late sowing. “Due to logistical problems following COVID, we could only sow in May 2023,” says Freitag. “We would ideally sow in early April.”

All four cover crops achieved more than 2.5 tonnes per hectare of dry mass. Two tonnes per hectare is considered sufficient to improve water infiltration and reduce soil erosion, and 2.5 tonnes per hectare is the level at which cover crops begin to suppress weeds. In contrast, the fescue control produced almost no dry matter.

Weed dry matter production was less than 1 tonne per hectare for all cover crops, but about 1.5 tonnes per hectare for the fescue control.

In 2024, both cover crops and weeds suffered a setback due to heavy rains in July and August. “It was too wet,” recalls Freitag. “Even the white clover on the farm was struggling.”

Nonetheless, the oats and the triticale-lupin mixture still produced more dry matter than the threshold required to suppress weeds and improve water infiltration.

“If I were a farmer, I would adopt cover crops just for the weed suppression,” comments Howell.

Other potential benefits

With half the data collection still to go, conclusions about the cover crops’ effects on the soil and the grapevines would be premature.

However, extensive work in vineyards (mostly wine grapes) by several researchers, notably the late Dr Johan Fourie, a respected researcher at the ARC Infruitec-Nietvoorbij and author of Cover Crops in South African Vineyards, showed that cover crops can:

1. Boost organic matterIn long-term cover-crop trials, Fourie demonstrated significant increases in soil organic carbon with several cover crops. For example, a rye cover crop more than doubled soil carbon (from 0.55% to 1.24%) on a sandy loam soil near Stellenbosch over 10 years.
2. Reduce fertilisationCover crops can augment the nitrogen available to vines, allowing growers to cut back on synthetic fertilisers. Nitrogen fixers, such as legumes, increase soil nitrogen directly, and other cover crops increase nitrogen indirectly by adding soil organic matter. Of course, excess nitrogen, especially when poorly timed, can be detrimental to fruit quality and storability (this was discussed in a December 2025 SAFJ article).
3. Improve water infiltrationWater droplets impacting bare soil can cause crusting of the uppermost few millimetres of soil. “Even though that crust is thin, it acts as a seal, preventing water from going into the soil,” says Howell. By protecting the soil surface, living cover crops and the mulch they create after termination prevent crusting and reduce run-off and erosion.

   Howell points to research by Kobus Louw, in which he showed that a living cover crop or a straw mulch decreased rain run-off by more than 25%, as measured over 34 rainfall events, and water run-off by more than 37%, as measured during a 46 mm irrigation event (applied at 12 mm per hour).

4. Save water and energyWhen mechanically or chemically terminated before bud break, cover crops form a mulch that reduces evaporative losses. Reduced irrigation demand saves water, as well as electricity for pumping it. Howell cites results generated by Dr Philip Myburgh on the effect of mulch on evaporation from the soil surface after irrigation.

   He reported approximately 8 mm of evaporation from bare soil for the first day after irrigation, compared with about 5 mm from mulched soil. After three days post-irrigation, daily evaporation from the bare and mulched soil was the same.

5. Protect fruit qualityWhile conducting a field trial in a table-grape vineyard near Keimoes, Fourie observed a heat wave’s effects during véraison.

   Bunch damage was significantly less in the treatments with a mulch derived from a vetch or oats cover crop (terminated before bud break) than in treatments without a cover crop or with dwarf fescue.

   “I see cover crops as positive in terms of all these benefits, and for sustainability,” says Howell. “Consumers want to see these types of practices.”