If they're in pots,they may be getting too much water;
zinnias like it on the dry side.

Hi, Mak. ;o) Sylvia could well be right, but let's play detective and see what more we can find out. Since you found no insects, we can pretty much determine that it is a nutrient disorder (eliminate mechanical injury, hmm?). Because it is located in the top of the plant and in new growth, we can eliminate all the mobile nutrients as a probable cause. This reduces the probability to either the secondary macronutrient Ca, or any of the micronutrients. All of these and Ca are immobile for all intents & purposes in your situation.

Though 2 or 3 micronutrients can affect leaf shape, Ca deficiency is almost always manifest in symptoms at the top of the plant. I was pretty sure what the deficiency was, but consulted the text "Water, Media, and Nutrition for Greenhouse Crops" for an exact quote/description of symptoms. It reads "Calcium Deficiency: Symptoms are expressed at the top of the plant on new growth. Young leaves may develop variable patterns of chlorosis and distortion, such as dwarfing, straplike shape, or crinkling. Shoots stop growing, petals or flower stems may collapse. Roots are short, thickened, and branched."

So ... if you concur that it is likely a deficiency, we next determine what could cause it. It could be caused by an actual deficiency of Ca in the soil, or very low soil pH, but the latter is not very likely. Waterlogged or poorly oxygenated soils can block the uptake of Ca and would be a distinct possibility. I know you've been lurking on the container forum, so you should be able to determine if this condition is likely. Anything that slows transpiration, like high humidity is another possibility, especially for your area. Finally, an excess of Mg (have you been using Epsom salts in your fertilizer program that could cause this?) in soils could cause Ca to be unavailable.

Does it sound like we're getting close?

Al