Dahlia

Katstez

Hi, I have a Dalia which when in flower is lovely large dark purple flower but it tends to grow really tall and the flower stems end up falling over. How can I stop this from happening. It is starting to grow now so wanted to catch it before buds start appearing. Thank you. 

Fairygirl

You just need to stake them, but ideally you do it before they get too big.
A few canes pushed in round the outside, with some string attached, will suffice, and the foliage will cover it quite quickly. 

KeenOnGreen

Sounds like it needs a Chelsea chop.  You can cut back about one third of the plant, either by cutting out selective shoots, and leaving some taller ones, or by reducing the overall height in a more even manner.  This will encourage a shorter, bushier plant, and it will lengthen the period in which it flowers.

fidgetbones

Stake it. Each strong stem will need its own stake.

JoeX

Pinching out each stem should encourage a bushier shorter plant

punkdoc

Staking is the answer.
Pinching out works while the plant is small. If you use the Chelsea chop on Dahlias once they have developed hollow stems, it will not help and may encourage the plants to rot.