Hi Lily,
Your GrowMore 20-20-20 looks like good stuff, with chelated trace elements, so it
should provide adequate nutrition to your potted zinnias. So it is a bit puzzling that apparent nutrient deficiency symptoms are showing up. There are several possibilities. The simplest is that you aren't using enough of the 20-20-20 GrowMore. But be careful. A big overdose of soluble nutrients can damage or kill plants. The "nominal" dosage of soluble nutrients, including GrowMore nutrients, is one tablespoon per gallon. It should go without saying, that is a
level tablespoon.
So my first recommendation is this. Mix up some one-tablespoon-per-gallon GrowMore and apply it to your zinnias as a foliar-feed drench. Use a watering can or equivalent to wet the foliage and let the run-off go down to the roots in the soil. If you want to apply that drench daily, then cut the strength to
one teaspoon per gallon.
If after a week of applying dilute GrowMore as a foliar feed daily, look for a noticeable improvement in your zinnias. If there are still problems, take more pictures and post them here. One factor that
might be in play here is that Zinnias prefer more Boron than most plants. But GrowMore contains Boron, so lets see what happens with the foliar feeds. After the first feed of one tablespoon per gallon, I would cut the strength to one teaspoon per gallon for the follow-on daily foliar feeds.
These are pictures of some of my indoor zinnia plants from last Winter.
I am starting some of the prep work for my indoor zinnia project this Winter.
ZM
