I have bamboo (Phyllostachys Nigra), which is growing well (very healthy green lea ves) but its stems are very spindly so when it rains, it ends up laying prostrate on the border and onto the lawn. Ive ended up staking it to keep it upright. If I dont stake it, it just flops again. Why is it so spindly? Its in good light, which isnt full blazing sun all day, but does get direct sun on it for up to 3/4 hours a day (when sunny!). I water it well, and when I planted it (two years ago), I planted it in good soil with good supply of well rotted manure. Havent fed it since. Do stems thicken up over time?