Allelopathy
Allelopathy is when a plant releases chemicals that stop other plants from growing nearby.
Think of it this way
Some foods put out strong smells that push other things away. A cut onion in a bowl makes the air around it sharp and sour. No other food tastes good near it — the onion's smell blocks them out. A plant does the same thing with its roots, sending stuff into the soil that stops nearby plants from growing.
Explaining allelopathy by grade level
Some plants make chemicals that keep other plants away. Bay laurel leaves hold something inside them that can kill weeds. The weeds stop growing and die. It is like the plant guards its own space.