Why plants bolt is a bit of a mystery, and may be a combination of factors. Most commonly I read/hear from gardeners that it happens when it gets too warm and these early-season plants see the heat of summer as their cue to flower. More reputable sources suggest that it is primarily triggered by either length of day or total length of daylight received (since sprouting), though some sites suggest that other factors like dryness of the soil also affects the timing.
Well, for whatever reason, my earliest plantings of greens have decided that it is time to bolt. I have bolting pak choi, rapini, arugula, and radishes(no bulbs). Since it has been such a terrible spring for growing, they are bolting while still tiny in size. Note that in the picture of pak choi plants below, even the tiniest ones with only two leaves are actually doing their best to make buds.
Happily, not everything has bolted. None of the lettuce or kale has bolted. The later plantings of all the greens still haven't bolted yet (adds credence to the total amount of daylight hours received theory). Interestingly, I planted two different kinds of pak choi at the same time. Only one of the kinds bolted. It was supposed to be a very fast-growing variety, while the slower growing variety is still going.
I just realized I could check back on my posts from last year to find out when stuff bolted then. It was at the very end of May, and I planted stuff probably a week or so later last year, so that also lends support to the total amount of daylight hours theory). Looking back at that post from a year ago, the difference in size of the plants at the same time of year is amazing. What a terrible growing year so far...