spirea suckers

Check out some of the barriers available commercially to contain the spread of bamboo. These are essential thin sheets buried in the ground, wouldn't need to go nearly as deep for spirea as for bamboo, but the cool thing is you don't lose any planting area since the barrier is very thin. Almost anything you just lay on top of the soil the spirea will find its way under or through, whereas sinking a deep concrete barrier is overkill and would alkalinize the soil in the process.

-paghat the ratgirl

You could use a barrier or you could walk the line with a sharp spade a couple times a year. If you use a barrier I doubt anything deeper than a foot is needed, leave a couple inches proud of the ground and angled away from the spirea to force any escapees up and into sight instead of down and under the barrier.

Thank you very much, that was most helpful