When to Plant Bamboo
The best time to plant will vary depending on many factors, including the region and the bamboo species. Even though my experience is with the mild climate of Western Washington, (USDA Zone 7a-8a) you can adapt this information to your region with some common sense.
Fall is the Best Time for Planting
For most regions, early fall (September through mid-October here) is the best time to plant bamboo. For one, the moderate air temperatures and light intensity reduce the demand for water, resulting in less stress on roots and foliage. Also, soil temperatures are the highest in fall, stimulating root growth and allowing plants to root out before winter sets in. In addition, the current season shoots will be woody by fall, eliminating the risk of shoots aborting or breaking during transport/transplant. Finally, it's a great time for the last fertilizer application of the growing season
Spring is the Next Best Time to Plant
Spring is another great time to plant (around here, late March-April). In spring, the warming air/soil temperatures, increased day length, and plentiful rain water provide good growing conditions for new plantings. In addition, spring is the best time to enrich the soil with nutrients the plant will use at the peak of its growth later in spring and early summer. During spring, container plants may have new growth emerging, so be careful not to snap the tender, new shoots when handling the plant.
Summer can be a good time to plant in milder regions, however, plants must be watered regularly during warm, dry spells. Smaller plants and new divisions should be protected from hot afternoon sun (which can be a real trick). In very hot, arid regions, you should plant only well established, larger container plants, and keep the soil constantly moist.
Around here, you can plant a healthy, well-established propagation (5 gallon container size, or larger) any time, except for a few days in December/Januaray when there is snow cover, or the ground is frozen. Remember that new divisions and smaller container plants won't be nearly as cold hardy, and they won't tolerate heavy, wet soils as well either. I keep small and new divisions in an unheated greenhouse over winter so I can control moisture levels, and protect them from cold snaps.
So if you can, plant in the fall to give your bamboo the best conditions of the year to establish and settle in for the winter. If you cleaned up on the "end of season" nursery sale, but missed the fall planting window, you can risk planting, or bring plants in a garage or cold frame during cold spells, and plant them in spring before the new shoots emerge. If you are planting in summer, regular water is important, especially in full sun exposure, or hot, arid regions.