Floppy perennials getting you down? There are several things to try before you throw in the towel.  First of all, selecting the right plant for the right space can prevent your garden woes. Read the label or search on the internet to be informed about the maximum width and make sure to account for proximity of nearby pathways and other garden features before you plant.  Planting your garden more densely will allow adjacent plants to support each other (and will also crowd out weedy opportunists.)  Plant shorter plants and ground covers around that will tolerate the taller plants shade.  Ensure light and moisture conditions will be appropriate as too much water and/or too little sun can result in leggy plants.  Shop around because there are often cultivars that are shorter/stouter or offer other advantages (for those of you who aren’t concerned with genetic diversity). 
Finally, perennials can be cut back at the same time they’re divided.  This will produce more numerous and shorter stems.  After your plant of concern has reached it’s maximum width cut back the stems and divide it (March through May or September through November for summer-bloomers and June through August for Spring-bloomers, although I suggest you research individual species first).  Leave some leaves so the plant can regenerate, of course. If you don’t like where the plant originally was, you may take this opportunity to transplant the divisions to another area or give them to a friend.  Cutting back your taller plants will result in shorter stems, but may shift bloom production to slightly later in the season.

Splaying stems of false sunflower, Heliopsis helianthoides.

If you are already encumbered by floppiness, a couple bungee cords wrapped around the stems is a quick solution, and affordable, since you may have these already laying around the house.
For more information and ideas about staking, check out this article: https://www.pdxmonthly.com/articles/2010/8/3/august-10-floppy-perennials-staking
Thanks for reading!
Irene Sadler, MLA, CBLP