Posted: 30/09/2013 at 08:42
I always support cosmos and bonariensis. It's a lomg growing season here, early too, so most things do get tall. I do 2 things. Firstly, in mid spring to late spring I cut back to make bushier plants. Then in early summer I support the main stems of bonariensis . My plants are (6' plus tall and stand up,to anything). Cosmos grown in the richer parts of my garden are also supported.
Come the autumn most plants tend to want to flop a little ..more moisture in the stems, still extending, etc.
No, I wouldn't add any potassium.
One point though......I,found that plants grown for the first time in a certain area will grow bigger but in subsequent years grow less vigorously. In a new border made last winter everything planted there has been enormous. I'm not expecting the same vigour next year