Problem with crops and plants.

Hi, i want to tell u guys about a problem with crops/plants and custom grass. so in one of my mod, i have a custom farmland and grass block, if i want to allow the custom farmland to make crops grow, i have to allow the: Can plants sustain on this block.by doing that, yes you allow crops to grow on you farmlands, but you also allow plants like flowers to sustain on the block. that the first problem, the second is: i have a new dimension in my mod, with custom grass, custom grass plant(herb) and custom flowers,if i want those plants to be generated in the dimension, i have to also allow plants to sustain on the block. so the plants can be generated, but u can also place crops on it, so it make the farmland useless. any way to fix this?