darnels, tares
稊 refers to darnels or tares, which are weedy grasses that grow among grain crops. The core idea is a plant that resembles a useful crop but is actually a harmful weed.
稊 combines 禾 (grain plant) with 弟, which likely contributes the sound. The character originally referred to a weed resembling grain, fitting the plant radical.
The grain plant radical 禾 on the left is paired with 弟 on the right. Imagine a younger brother (弟) among the grain stalks—he looks like part of the crop but is actually a useless weed.
For テイ, picture a tea plant (ティー) growing among the grain, but it's actually a darnel weed: tea -> テイ, and the weed is hidden in the crop.