Hydroponics produces some pretty beefy stems. At least on my pepper plants. I attribute it to the very rich nitrogen I was giving the plants. Try some high nitrogen fertilizer ONLY WHEN THE PLANTS ARE OLD ENOUGH, about 3 weeks in
once you're ready to transplant into a pot (when you have ~4 sets of the signature marijuana leaf), you bury it all the way up to the first pair of real leaves. the buried stem will grow roots and develop stronger than just using the plant's natural root