That only applies to the plants you sow yourself. You have to invest points in "Living off the Land" to produce seeds and prepare the soil with a hoe.
You can harvest them e.g. with your fist by hitting them and leaving behind a seedling. But this only works when the plant is ready. If it is still growing or you hit a seedling then you get the seed in your inventory. You can see in the tooltip if the plant is still growing.