You can buy the fastest growing bamboo in the world but if you want it to get big, faster, follow the procedures below!

When I got serious about collecting bamboo there was a time when I planted many species each year, often in multiple locations. I wanted lots of bamboo fast. I live near a horse farm. I was able to get free manure (mixed with straw) and placed a 2 to 4 inches of this around each planting, usually 3 to 4 feet in diameter the first year. I might add some artificial fertilizer such as 8-8-8 spread over the area. I would cover this with straw or leaves or whatever was available. When fall came I raked up a very deep mulch around each planting, at least a foot or more of leaves in a 5 to 10 foot circle. If planting a fence row I fertilize and mulch in a long row connecting the new bamboo plantings, however thick I wanted the screen to be.
The newly planted bamboo will start sending out rhizomes which will divide and spread so the bamboo is eventually going in all directions. However it may spread much faster in some directions. You may be able to control the spread somewhat by fertilizing in the directions you want the bamboo to go, as when planting a row of bamboos for screening.
In the winter/spring of the following year I would spread manure again, this time in about a ten foot circle with a new layer of leaves or straw and would follow this procedure for at least a couple of years, often 3 to 4 years. I did this for most of nearly a hundred plantings spread over several years and acres. I could drive my truck up to or even around most all the groves, especially when they were young so it was fairly easy to spread the manure with a shovel. After 2 to 3 years I usually stopped with the manure but continued with 8-8-8 or even ammonia nitrate. I also continured to rake up leaves for a mulch but it doesn’t take too long for most bamboos to self mulch. Soil ph should be neutral to very slightly acidic for most bamboos. Lime acordingly.
When I was following these procedures I was giving bamboo to my brother and sister who also had land to plant it on. They planted and watered and that’s it. After 5 years all my groves of the same species were at a minimum twice the size, usually even larger. Many more canes, much larger canes. I think at a minimum you should mulch heavily with leaves for a few years. It will cut down on weeds while the plants are small, protect the plants in the winter by insulating the ground, and as they decompose they will build rich soil. The mulch will also help keep in moisture during hot periods. When the groves were young and the manure and mulch were thick, the soil below was just alive with worms and was damp in the hottest periods. The mulch and manure together decompose fairly rapidly.
My largest grove of bamboo, in the most choice location is Moso bamboo, the largest cold hardy bamboo. I drove to Anderson,South Carolina to dig 5 nice plants to start the grove. I planted these in a circle and spread a truck load of horse manure. I did this for the next few years and the Moso responded accordingly. It grew deep green and put up more shoots than I expected each spring. Today this grove has beautiful 5 and 6 inch canes that cover an area 50 feet in diameter.
I’ve seen this over and over. I planted P.vivax, the next largest cold hardy bamboo and gave it this treatment. In less than 10 years I had multiple 5 inch canes, like a small forest. I planted P. vivax aureocaulis in an odd place that never got the manure and mulch. The difference today is amazing. The Green Stripe Vivax has a few 4 inch canes, most are smaller.