Answer: Most perennials should not need staking. Those that do not normally need it but flop can be caused by being grown in less light than they prefer or if they are overfertilized, so be sure you are planting them in appropriate spots and do not overfeed them. Some perennials will also flop or spread out if they need to be divided. Some can be kept denser and a bit shorter than normal by pinching, these would include the sedum you mentioned -- although normally with sedum flopping indicates too little light and/or a need to be divided. Mums and asters are traditionally pinched, you could also do this to boltonia and vernonia, for example. Mums are usually pinched beginning in early spring, and repated when they grow a few inches, through about the fourth of July. Sedum would be pinched or sheared off once in mid spring. The asters are usually pinched several times, boltonia and vernonia maybe once. The pinching delays flowering -- in the case of mums the repeated pinching delays it until fall. Increased number of blooms can reduce individual bloom size somewhat (this is very true for roses) but there is a tradeoff in quantity and having a stockier plant. Another aspect of this is that many early blooming perennials will rebloom if deadheaded or cutback after the first bloom flush. The basically is a form of pinching or you could say pinching is a form of pruning.
Q&A Library Searching Tips