If that was my area: Visit the local plant nursery and buy a 6-pack of plants. Read the label carefully to see how tall they are (I noticed both 12" and 30" earlier this month). Then buy a packet of seed of the opposite height ~ if the plants are 12" at maturity, buy seeds for tall ones or vice versa. If there is a choice of heights available, I'd prefer 8" and 20". Well-spaced zinnias will be bushy while crowded plants tend to be skinny and floppy. Bushy 20" plants will hang over the sidewalk but not too much, whereas a 36" plant may be obnoxious to walk past. Also, 24" and shorter plants don't need to be staked, but I always end up having to stake the taller ones in July; unstaked tall zinnias have a tendency to flop and wind storms will push them over.

I will assume you have done at least a minor prep of the soil ~ zinnias will grow in poor soil but they do much better if there is organic matter worked into it. Space the plants evenly the length of the bed and water them in (that means give each plant a couple cups of water immediately after tucking into the new home). Read the label on the seed packet to get the recommended distance apart. Make a mini-row between each plant and put the seed in, just a tiny bit closer than recommended: seed for 12" plants will probably recommend 6" apart, plant at 2 inches and remove the extras after they sprout. Zinnias sprout very quickly, usually within 7 days; and bloom within a few weeks. Buying a flat of plants just gives you something to look at while waiting.

As soon as the seeds sprout and have 4 leaves, do the no-weed-mulch thing: water the bed deeply (that means you can stick your finger into at least 2" damp soil), and then pull out any weeds already growing. Now tear some old newspapers into smallish pieces, dunk into a bucket of water and cover all of the bare soil with a layer of wet paper (best is 4 to 6 sheets of paper thick). Now top the newspaper with either finely shredded leaves or with cocoa-bean shells; use a generous inch of leaves, or enough shells that paper can't be seen. It is recommended to thoroughly wet the shells after they are spread; no more water is needed if leaves are used. With this mulch the only weeds that will appear will be from airborne seeds and easily pulled up while tiny. Well-spaced plants will become bushy and provide some shade, but it is more efficient to prevent weeds during the growing period from seed to full-grown as well as during the rest of the summer. To be honest, I detest weeding and will mulch *very* carefully (so as to not cover the rows) immediately after planting the seeds.

BTW, both 'State Fair' and the cut-and-come-again varieties make rather tall plants ~ usually 4 feet tall in my garden. They make loads of flowers but *must* be staked, and even then will often topple in a stiff wind.

link is to a company I have dealt with and like; Zahara is a very good zinnia which truly does seem to be mildew-resistant. In general, problems with mildew will be minimal if you are careful to not wet the leaves when watering (a soaker hose is wonderful) ~ and yes, additional water will be needed during dry spells.

Here is a link that might be useful: Zahara zinnia

Wow! Thank you for such a thoughtful, information-packed response! I appreciate it. :)