My seedlings out in the front yard are still pretty small, and I'm thinking that they are slow partly as a direct result of cool weather but also as an indirect effect. The indirect part would be that the nitrogen from the organic fertilizers is not yet available. It needs to be "set free" through a decomposition process, and the cool soil may be slowing that down. That's my current theory, anyway.
The nitrogen in that soil right now is in the form of some cottonseed meal and a couple of bags of "humus & manure" from a store. Given some more warm weather and a bit more time, that nitrogen will become available for use by my plant babies, but I am impatient. I will probably water that bed with some fish-emulsion in a dilute solution tomorrow. That form is more readily available to plants, but when mixed according to package directions won't harm my little lettuce, chard, spinach, mustard, carrot, beet, and scallion babies.
My garden doesn't usually have this problem, but it has happened before, and the fix was liquid fertilizer, so the fishy plan is likely to work. Gardeners who rely more on bags of conventional fertilizers will not be so constrained by the weather, which is certainly something to consider when planning a garden, but I'm sticking with my stinky organic amendments.