I've recently grown Pumkins (shirimen) in a solution that is close to mod. Hogland. I have set the seeds in one of my running devices. The seeds have germinated fine in a 1.2 EC solution and have grown nicely. Root developement was quick and healthy as well. This clearly tells me that one can add fertilizer a.s.a.p. PS: I was not using rock wool, but a mix of rough sand, perite and rice hulls.

Traditional or classic methods tell to grow in water and give week fertilization first, to increase root developement. I don't actually agree with this strategy. Perhaps roots build-up a little quicker, but the seedlings have a huge nutrient requirement from start. If they lack nutrients, they grow slower! Some people grow pumpkins or cucumber in high fertile compost, some put the seeds directly in compost as well. In a natural soil environement, there is no such thing as gradual nutrient raise. A seed germinates and the plant starts to grow at a specific place and soil. The content of nutrients in that soil is also the same from start to maturity (well, in fact and mostly a little lower as in hydroponics). But actually it rather tends to decrease with time. The more the plant grows, the more it consumes the nutrient content of the surrounding soil.