I usually plant my transplants outside towards the end of May (usually around May 25) in zone 6a. The transplants are about 6-8 inches tall with good robust stems when I put them out. It takes about 8 weeks for them to get this big, so you can start your seeds in early to mid April. Before you put them in the ground, make sure you put them outside for several days (at least during the day, I often take them in around 10pm to avoid the coldest part of the night for the first couple of days, or if there is unseasonably cold weather predicted). This process is called "hardening off" and gets your plants used to the outdoor environment.

If you start seeds indoors, make sure you have some supplemental lighting for them. A sunny window will often cause them to be tall and leggy without good support or well-developed leaves. Four T8 fluorescent bulbs held a few inches above the plants will work better, put on a long day photoperiod (16 hours of light at least). Although seed catalogs sell expensive set ups for this, you can get fixtures and bulbs for much cheaper at the hardware store. Here is a helpful article with more info from the show "You Bet Your Garden" which is on public radio: http://www.gardensalive.com/article.a...