We never recommend plastic "barriers" for bamboo plantings. In the first place bamboo obviously does not want to be contained or one would not be considering containment.
To see our discussion of plastic "barriers", please CLICK HERE.
We seldom even recommend metal water troughs, concrete barriers, or planters built either above ground or partially submerged. Planting bamboo in any type of barrier will not replace the required maintenance that will create a healthy grove of beautiful bamboo.
That being written, the classic berm method is one of the easiest and effective methods of containment. The bamboo is simply planted above ground level in a berm 12" - 18" high. When the bamboo tries to expand horizontally you merely clip off the end of the rhizome. This berm method would not suit many gardens, and really does not address what makes a healthy grove. Thinning, fertilizer, water, good soil, drainage, and the occasional digging of a stray rhizome.
If you do decide to use a plastic barrier you must have a plan to maintain the grove in any case, so what is the benefit of the barrier? The maintenance is not too much different than if you had no plastic at all, perhaps easier.
One way to maintain a grove is to dig around it, at least once every year, in the late fall or winter, and prune the rhizomes back to the grove.
There are many issues with trying to use plastic to contain bamboo. The largest issue is the containment method itself. Bamboo plants grow rhizomes horizontally, under the ground, at the same speed as they grow shoots above ground. These rhizomes can grow at any angle from the grove. When they encounter a barrier of any kind they stop, split, and go both directions, usually following the side of the barrier. Again, here is an in depth discussion: CLICK HERE.
Another issue is that the containment material must extend above the ground. This causes three problems, one is that people tending the bamboo grove step on the plastic and it tears. Even fine tears can begin to weaken the containment.
Another issue is that the rhizomes, roots, and emerging culms tend to push up the ground in the containment, causing stress on the plastic edges. The third issue is that it is just unsightly to have a thin plastic sheet sticking up in the garden. If plastic is used a garden edging could be installed, stapled or glued to the top of the plastic sheet to prevent tearing.
Another issue is that rhizomes are going to "crowd" the containment. It is best to trim these back and keep them away from the barrier You must be very careful not to create a breech in the barrier when you trim.
Another important issue is that you choose the right bamboo in Seattle, one that is not too aggressive or invasive. The size factor plays a part in containment as well, bamboos such as Phyllostachys rubromarginata may be hard to maintain in a containment system that is less than 500 square feet. Phyllostachys nigra is a bit easier to maintain in a smaller containment, just as examples.
To contain a running bamboo at planting time or to contain a grove already in your garden, dig a trench completely around the clump’s intended limit. Cut off any unwanted rhizomes on plants that have already spread too far. The roots of bamboo plants do not extend deeper than about 2 feet, so a 3-foot-deep trench will suffice.
A berm method is a great choice and we can consult and discuss this option when we look at your garden area. The berm should be 12" to 18" high and the bamboo planted on the top of the berm. The height of the berm is a natural containment for the plants and allows you to "harvest" rhizomes that can be potted and sold, given away, or replanted elsewhere.