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A container plant can eventually fill much of its pot with roots. When roots become densely crowded, they may begin circling around the container instead of spreading naturally through fresh growing medium.
Water can then move differently through the root ball, and growth may slow. One clue is unusually rapid drying after watering, although different plants behave differently.
Repotting or appropriate root pruning can restore space for healthy root growth. The leaves may look fine for quite a while before the crowded root system becomes obvious.
© Royal Horticultural Society