Rooibos Fields Run on Winter Rain. One Farm Now Irrigates at Scale.

Rooibos seed is sown in irrigated seedbeds, but once the seedlings are transplanted into the field they "do not receive any additional water", a 2015 University of Cape Town doctoral study states². Mean annual rainfall across the growing area ranges from 180 to 600 millimetres (about 7 to 24 inches). That single fact decides most of what follows in how the red bush is farmed: where it can grow, how it is planted, and how exposed the whole industry is the moment a rainy season falls short.

2. Lotter, Potential implications of climate change for Rooibos (A. linearis) production and distribution in the greater Cederberg region, South Africa, PhD thesis, University of Cape Town (2015)

The growing area gets most of its rain in winter, June to August, and is arid through the summer months, the UCT thesis records². Aspalathus linearis is planted in deep, coarse, acidic sandy soil that drains fast and holds almost no water, exactly the opposite of what most crops want, and it is planted there anyway because that soil is what the plant evolved for. A grower cannot decide to water through a dry spell the way a vegetable farmer would.

2. Lotter, Potential implications of climate change for Rooibos (A. linearis) production and distribution in the greater Cederberg region, South Africa, PhD thesis, University of Cape Town (2015)

What has been measured, on young plants under real field conditions, tells a more specific story. A 2023 Stellenbosch University study, published in Scientific Reports, tracked rooibos seedlings for fifteen months and found the taproot reaching only about 65 to 71 centimetres (26 to 28 inches) deep by that point, the peer-reviewed study reports¹, noticeably deeper in deep soil than in shallow soil. Alongside the taproot, the plant also grows a second, shallower set of cluster roots, tightly packed within about 10 to 30 centimetres (4 to 12 inches) of the surface. That is an unusual place to put root mass in ground this dry, and the same study proposes a reason.

1. Van Schalkwyk, Hoffman, Hardie & van Zyl, Soil water dynamics and biomass production of young rooibos (Aspalathus linearis) plants, Scientific Reports 13:15154 (2023)

The researchers tracked soil moisture and temperature hour by hour through that shallow layer. When the soil there cooled to its lowest point of the day, about 25°C (77°F), water vapour moving up from the wetter soil below condensed at that cooler layer instead of simply evaporating away. The study puts the condensation at an average of 0.33 millimetres a day, or roughly 186 millilitres (about 6 fluid ounces) for a plant drawing on a patch 0.75 metres (about 2.5 feet) square, the paper calculates¹.

1. Van Schalkwyk, Hoffman, Hardie & van Zyl, Soil water dynamics and biomass production of young rooibos (Aspalathus linearis) plants, Scientific Reports 13:15154 (2023)

The same study put a number on why deep soil matters so much to a grower choosing a field site. In the 2016/17 growing season, an unusually dry one with only 98.9 millimetres (about 3.9 inches) of rain, plants in shallow soil ran completely out of usable water after 168 to 174 days. Plants in deep soil held on until day 225, the same paper reports¹, nearly two months longer on the identical rainfall.

1. Van Schalkwyk, Hoffman, Hardie & van Zyl, Soil water dynamics and biomass production of young rooibos (Aspalathus linearis) plants, Scientific Reports 13:15154 (2023)

The UCT thesis also tested how much a young plant's own defences could stretch. It found real adaptation under drought stress, tougher leaves and more efficient water use among seedlings put under pressure, but those adaptations "were not able to offset more severe conditions in field settings", the thesis's own findings state².

2. Lotter, Potential implications of climate change for Rooibos (A. linearis) production and distribution in the greater Cederberg region, South Africa, PhD thesis, University of Cape Town (2015)

At Klipopmekaar, an organic Cederberg grower spent years researching and trialling supplemental watering before rolling out what it describes as the first large-scale supplementary irrigation of rooibos anywhere in the industry, the farm's own account states³. The system is solar-powered and closed-loop, and designed to lose almost nothing to evaporation.

3. Klipopmekaar, Rooibos And Climate Change

First, it is supplementary. Second, Klipopmekaar reports a longer working life per field, steadier year-to-year yield, and better quality³.

3. Klipopmekaar, Rooibos And Climate Change