Host tissue penetration occurs to the point of vascular connectivity. Until the Striga emerges from the soil, it is entirely parasitic, but when it rises out of the ground, it is only partially parasitic, as it is able to then photosynthesize its own food but needs water and minerals from the host. Host specificity means that a parasite identifies and parasitizes only a particular kind of host organism. In germination, Striga require the aid of a chemical stimulant, which is available in the roots of the host organism. Witchweeds also have the ability to recognize specific genotypes in host roots.
Biologists believe that host specificity is most likely a by-product of natural selection.
For example, the species S. asiatica prefers grasses because it is native to the savannas. Evidence for non-specificity is shown when crops are changed. For example, when a farmer changes crops from sugarcane to corn, those species of witchweed that are specific to sugarcane may be almost eradicated, but in a few months to a year, a new rise in Striga occurs, this time with specificity to corn. Evolution has dictated that witchweeds should have this kind of adaptation in order to survive as the relative abundance of host plants changes. Once the Striga plant establishes itself in a field, it is difficult to remove.