Answer: As you have discovered, any roots remaining in the soil have the potential to grow into a new bamboo plant. While vegetation killers such as Round Up and Brush B Gone can kill the tops and the roots of plants to which it is applied, there will always be roots without tops, just waiting underground for a chance to pop up. You can dig every last root out or you can diligently cut off every sprout you see, before they produce leaves. If you do this you will eventually starve the roots (no leaves, no photosynthesis) and you'll be rid of the bamboo.
Since you live in a warm region I wonder if soil solarization will work? It might be worth a try. Solarization consists of covering the soil with a clear plastic tarp for 4 to 6 weeks during a hot period of the year when the soil will receive maximum direct sunlight. When properly done, the top six inches soil will heat up to as high as 125 degrees. Over several weeks, that's hot enough to kill a wide range of soil inhabiting pests such as; wilt and root rot fungi, root knot nematodes and noxious weeds. In addition, solarization stimulates the release of nutrients from organic matter present in the soil.
Large sheets of plastic to use for solarization are available at hardware and home supply stores. Use clear polyethylene, not black plastic! Clear plastic produces higher soil temperatures faster than black plastic. Sunlight passes through clear plastic to heat the soil directly. Black plastic intercepts the light, and soil is heated primarily by conduction only where the plastic actually touches the soil. Some of the heat generated when sunlight hits black plastic is lost directly to the outside air.
Before applying the plastic covering the soil should be moistened. Wet soil conducts heat better that dry soil. Moisten the soil to a depth of one foot. This will enhance heat penetration through the potential rooting profile. Also, in moist soils, pest organisms are more active and thus more susceptible to the lethal effects of heat.
To install the plastic over the garden area to be solarized, first dig a trench around the garden perimeter. The trench need only be about 8 inches deep and 12 inches wide. It will be used to hold the plastic in place.
Lay the plastic out so that it covers both the garden area and the surrounding trench. As the plastic is pulled tight, hold the plastic in place by covering it with soil in the trench. As you move around the perimeter covering the plastic, keep pulling it tight. A tight fit against the soil surface allows for better heating. Burying the edges will also prevent the wind from picking up the plastic tarp and blowing it off.
Leave the plastic covering on for a minimum of 4 to 6 weeks.
Wish there were an easier solution and I hope this information is helpful!
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