Im just saying that you could have "a pot of soil" that you could plant a seed in. Maybe water it a few times, over the course of a few earthen days, watch it grow into a full plant. It squirts out a few of whatever, maybe some more seeds, and bam, there it is. You can just as simply place them in your house, or in the yards of your house.
That way your not completely excluding free accounts the fun of growing crops, but if they want to do the big stuff "fruit trees", they have to have a house.