Many times containment is a second thought for people who have bamboo planted. Sometimes that’s OK since a pruning trench or rhizome barrier can be installed a year or two after the bamboo planting with good results and minimal, if any, rhizomes escaping to unwanted areas. However, I recommend that anytime you’re installing a running bamboo, you should either install containment at the same time or have a plan in place.
One containment technique that can only be installed at the time of planting (or before) is the raised bed. This works on the same principle as the pruning trench: rhizomes are naturally shallow and grow horizontally. If they reach a vertical drop, either straight down or very steep, they’ll come out of the side the soil, thus exposing themselves, before they dive back in. With a pruning trench, which is a 12” deep, ~5” wide trench around the bamboo, the vertical drop is achieved below ground level. But you can also get this same result by created a raised mounded bed above the soil line.
There are two important factors when using a rhizome-pruning method of controlling bamboo: proper installation, and proper maintenance. For a proper installation, we’re looking for at least 12” of vertical drop within 12” or less of horizontal span. IE, a 45-degree angle or steeper. You can get this by simply mounding up soil and packing it down on the side to achieve the angle.
I recommend using at least a 2” shallow trench as an extension of the sloping soil to mitigate any wash out the new soil may have during the first years of rains. You can also use a deeper trench and shorter bed, as long as you have at least 12” of combined drop.
The raised bed technique is useful in areas with a lot of existing roots, utility lines, or other obstacles to trenching. It also can give your initial bamboo planting some added height, effectively speeding up the fill-in time for something like a privacy screen.
During installation, the first (lowest) layers of soil to be brought in should be heavier with more clay in them. As you build the bed up, use lighter and richer soil, topping it with pure compost and then hardwood mulch. What you’re trying to do is make the conditions favorable to the rhizomes at the top, and not favorable on the bottom. I usually recommend a single- or double-shredded mulch on raised beds since they will control erosion better and last longer than a triple-shredded mulch.
Maintenance for a raised bed is the same as it is for a pruning trench: walk around the edge 2-3 times per summer, and 1-2 times per fall, and once in the winter and spring depending on the weather. If any rhizomes have shown themselves, simply clip them with a pair of hand pruners. If you wish you can then pull the rhizome out on the outside of the bed, but this is usually unnecessary since it likely isn’t mature enough to survive on its own.
During maintenance, you’ll likely need to rake out the shallow trench of any leaves or other debris that may have filled it. If you are noticing rhizomes near the bottom, you may want to deepen your trench by an inch or to be sure nothing tunnels below. Some rhizomes can go deeper than others, depending on the bamboo type, soil type, and how much water is available on the top.
Speaking of watering, the best practice it to keep it moist on top, but not necessarily let the water soak deep. This is the opposite of conventional wisdom for most plants. Remember, rhizomes tend to follow the water, sun, and organic matter. A soaker hose set to a very short duration (try 5 minutes to start) every day or every other day is a good option, depending on the time of year and how much direct sun it gets.
For small beds less than 12’ long, you may want to perform maintenance more often, at least at first, to make sure your bamboo is spreading into areas you want it.