Bushiness is not the growth habit of Zinnia. And I think that pinching, which would be done to get branching, would be done after seedling stage.

Yes, pinch them when they're small. As four mentioned, zinnias don't really have a bushy habit. A couple pinches would be fine.

Hi guys,

I would wait until they got at least two big leaf pairs. That would give you four side branches. For more side branches, delay the pinching until more leaf pairs have formed on the main stem.

ZM

I'm late on this, but same as Zen_Man, although it depends on the zinnia a little. Small guys, pinch a little earlier at the two big leaf pairs. Big guys, maybe even a little later. And I always pinch again after each bloom fades; they tend to re-split (one informal name for Zinnia is Cut-And-Come-Again).

I think the Cut-And-Come-Again name refers to a particular strain of zinnias, and that name implies that actually removing its bloom will encourage more blooms. That technique of cutting off blooms can be applied to any zinnia strain to make the plant bushier and have more blooms. Don't forget you can see larger versions of photos here in Houzz by clicking on the photo. You can close that larger photo by clicking on the X in the upper righthand corner.

ZM