If the plants are spindly, they are not getting enough light. Do you have a fluorescent shop light you can put them under? (suspended 2-3 " from the plants) All of the dahlias growers that I know, divide their tubers every year, this will ultimately give you more flowers, because it gives you more plants. Pinching the tops of the plants is standard practice. It is usually done after the 3rd or 4th pair of true leaves. Pinching encourages lateral branches to grow out, plus causes the top of the plant to fork. The laterals, and the two forked branches should all develop flowers. So pinching gives you a bushier plant with lots more flowers. Another standard practice is to only allow one central stalk to grow from each tuber. This give you a stronger central stalk, which produces larger flowers. If you are not concerned about size then you can allow multiple stalks, but on the larger flowering types, it can make it difficult to stake the plant. Myself - if the normal bloom size of the flower is 3-4" or less, I let multiple stalks grow, otherwise I snip off additional stalks. Hope that helps.

When you replant in the spring and the sprouts start coming up, pinch off all but the largest sprout. When that single spout is 12-18 inches tall, pinch off the center candle. This will cause the plant to be thicker, bushier, and have thicker stems. You will have a 3-4 feet tall plant instead of 5-7 feet with thin stems.

There is no one correct way to grow tomatoes and so much is dependent on your season length, soil fertility and summer heat and humidity levels, and crop goals. I grow on cattle panels cut to fit my 4'x12' foot beds. The extra 4 ft length which is cut off ends up with spikes on the bottom, and works great to push in the ground for trellising cucumbers. Or you can just let the extra overhang each end of the bed - and the end vines can be trained to use up the space. My husband did complain it was hard to mow grass under the overhanging part.. Or you could even put a large potted tomato at each end of the overhang if you don't have heavy duty bolt cutters to do the job. I put 7 plants per bed, so my spacing is quite close. My panel is set in about 1/3 of the 4' width on the bed, and tomatoes are planted next to the wires. I only grow one row of tomatoes per bed, the opposite side of the bed is for bush beans, broccoli, or some short crop. The raised beds are 3ft apart, so that's about a 7ft spread between rows of tomatoes. I also have some 8 ft raised beds that I normally grow 5 indet plants along one edge, if I am growing dwarfs, I jam more in. I don't recall how wide you said your beds were, but If you plan on putting tomatoes along both long edges and it's too narrow, you could have trouble getting in to pick the inside tomatoes, as you get a wall of foliage. I don't prune much, except to remove stray suckers from entering the footpaths between beds. I live in a shorter season area, so I don't worrry much about the plants outgrowing their supports, they just flop back down. I don't have the constant extreme humidity that folks in the south have to deal with, I might prune for better air circulation if I did. Some tomatoes have very flexible branches, and I just push and weave them through the openings of the wires. The stiffer branched varieties are tied to the wire as needed with whatever baling twine or rags I have around. Never fails, every year I snap off beautiful early flowering vines or suckers by being impatient and trying to bend stiff stemmed varieities through the panels holes instead of walking back to the house to get something to tie them with.

With most tubers, I end up with 2-6 stems that try to grow, but I normally pick 1 healthy one and remove the rest. But I wonder if it would be a good idea to allow 2 or 3 to grow in order to create a thicker plant? Next, what if I planted 2 tubers fairly close together and allowed them each to grow 1 healthy stem so that I end up with a thicker dahlia 'bush', with twice as many flowers?