Karewa soils, which cover approximately 2500 square kilometres in the Kashmir Valley, are prone to differential settlement under loading conditions, necessitating pre-construction soil enhancement to ensure structural stability. Capsella bursa-pastoris, commonly known as shepherd’s purse, is a prevalent local weed in the Kashmir region that is often discarded as waste material. This plant ash has been used to stabilize the Karewa soil. The plant ash was incorporated at varying contents from 0 to 15% of the soil dry weight in increments of 5%. The experimental results reveal that the addition of plant ash increases the maximum dry density and reduces OMC. The stress–strain behaviour of the modified also changes significantly. Peak stress is found to increase, and strain corresponding to peak stress is reduced significantly. The swelling index of soil reduces with the addition of ash. The results of experimental testing are encouraging and reveal that the ash of Capsella bursa-pastoris can be a good alternative to improve soil characteristics, and at the same time, it increases the brittleness of the soil sample.