Zinnias, Cosmos, Marigolds, Celosia etc: Cut the stem just above a joint. Gently remove excess leaves. Toss the scraps into the flower field. They will "self-compost".
Sunflowers: Visualize the height of your vase, and add a little extra before you cut the stem.
ZINNIAS: Give them the "wiggle" test. Select a zinnia stem that is mature, and has a stiff neck. Mature zinnias show at least part of a gold ring around the center. Gently rustle the stem at the "neck" below the flower head. If it is firm, this will be a good flower. Don't pick zinnias that still have tender stems, they will break easily.
COSMOS, MARIGOLDS, other small flowers: Select mature flowers here also, ones with sturdy necks. If the neck of the stem is not stiff when you cut it, it will Not get better. Pick quality flowers. Keep the water in your vase very clean! Add a drop of bleach to the vase water.
Keep the water in the vase very clean, and trim the stems every few days. Add a drop of bleach.
Cold temps can slow their maturity and blooming. Excessive rain can stunt their growth. Daylength & climate factors affect their height. Seed availability affects which sunflowers we grow.
Sunflowers don't grow well in the shade. Sunflowers like it hot! So do zinnias. Sunflower plants "follow" the sun while they are green and growing, but they stop this when the flower opens up.
Drop any clippings back into the flower patch. They will decompose & add nutrients back into the soil. Doing this will help feed the next crop. Thank you!