Answer: You have asked an interesting question. In my experience first year perennials from seed which manage to bloom may be a bit shorter than the mature plants, but not that much shorter. The plants are usually so busy establishing themselves and gaining bulk that they don't bloom or, if they do bloom, they don't bloom much.
So when that happens I wonder if they have been stressed somehow which caused them to put energy into blooming rather than into establishing good roots.
Interestingly enough, however, the delphinium can be grown as an annual in warm areas, so it is an early bloomer if started early enough in the season. (It will be shorter in full sun, taller in partial shade.) This plant needs are very deep very rich soil and plenty of moisture to do its best; in this case I would suspect a possible soil problem, either not prepared deeply enough or just not quite rich enough or just not quite moist enough. For the daisy, I would suspect a similar problem. For the yarrow, however, I might suspect a soil that is too heavy. These plants do best in full sun on a fairly lean soil that is light and well drained -- a sandy loam is perfect for them.
Finally, you might wish to dig one up and check the roots. I have seen plants which had been potbound prior to transplanting, or which for some reason simply did not root out into the surrounding soil and they performed much the way you described: sort of a "bonsai" effect if you will.
Finally, you might wish to run some basic soil tests to check your soil and find out for sure, since perennials vary so widely in their preferences and requirements. Your County Extension (tel. 424-9485) can help you with the tests and with interpreting them.
Congratulations on your first year's success!
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