Weed-reading: Kilifi farmers improve yields by learning from invaders

Kilifi farmers have found that weeds, like crops, each prefer different types of soils, meaning they can read the weeds in knowing how and where to plant to get the best food crops.

Amina Samba from Riata village in Taita Taveta County, uses nut grass (Cyperus rotundus), locally known as ndago, to spot the best places to plant food.“In a place where you find this type of grass infestation, I know that if I plant any food crop, it will definitely do well,” she said. 

Her wisdom is borne out by science, with Weeds and What They Tell You by Jonathan Grieve reporting that nut grass thrives in sandy to loam soils with good soil structure and moderate moisture levels, as well as moderate to high fertility, low salinity, and mild acidity.

This kind of soil is ideal for crops such as maize, beans, and cowpeas. 

However, its deep roots can make it hard to clear, and its presence means the soil is short of calcium, phosphate, and zinc, which need building up for top crop performance.

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Another strong marker of fertile soils is donkey grass (panicum trichocladum). 

In Ganze, Kilifi County, the women’s group Ukoka, led by Shida Kenga Nasibu, relies on donkey grass to mark out ideal crop areas. “In a place where we see it growing, through experience, we know the area has a lot of fertility,” Shida said. In fact, donkey grass thrives in fertile, well-drained loamy soils with a pH of 6 to 7.5, making it a clear indicator for vegetable and maize farming.

Dzadza, or wandering jew (Commelina africana) is also common in Kilifi and a marker of good fertility. According to Neha Singh’s Ultimate Guide to Wandering Jew Plants, this weed grows in slightly acidic, well-draining soils with a pH of 5.0 to 6.0.

Another contender as a weed sign-post, is blackjack (Bidens pilosa).

“Where blackjack grows can indicate that the soil is rich in nutrients. The food crops here—millet, sorghum, maize, and cassava—always do well,” said Fred Barasa of Busia County. 

Blackjack thrives in soils with a wide pH range, from 4 to 9, but additionally mobilises critical nutrients like zinc and boron through its roots. However, its presence can also signal the overuse of herbicides like glyphosate.

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Ngumbao Kingi uses pigweed (Amaranthus) as a soil health gauge. Known as “mchicha” in Swahili, pigweed grows in compacted soils with high clay content and indicates high potassium and nitrogen levels. Studies show it thrives at a pH of 6.4, though it may also signal deficiencies in phosphorus and calcium.

In Nyanza, David Oduol relies on black nightshade (Solanum nigrum), also known as “managu.” “Black nightshade can be seen in areas that always give good harvests for sorghum and maize,” he said. The weed thrives in soils with high nitrogen and phosphorus levels, preferring slightly acidic soils with a pH of 6 to 6.5.

Research from the Kenya Climate-Smart Agriculture Project (KCEP-CRAL) confirms that blackjack, pigweed, and black nightshade all tend to flourish in fertile soils. If you have a weed that seems to grow where one crop or another crop thrives, please tell us at FarmBizAfrica.com here and we can add it to our forthcoming weed-reading guide for African farmers. We would be happy to credit you with this insight, so do also tell us your full name and where you are farming.