I have a fairly large Spirea bush about 15-20 yrs old. In the fall of 2012 I cut it back to ground level in the hope that it would kill it as I would like to replace it with a weeping Japanese Maple. In the next month I applied roundup to any new growth. Likewise in the spring of 2013 I applied roundup until about July when it was just too hot for gardening. Since then part of it has comeback to about 28″ tall. Do I stand any chance of getting this out of that space.? There are Hostas in the area which I want to keep. Your suggestions/thoughts would be appreciated.
Location – North Okanagan of BC
False Spirea (Sorbaria sorbifolia) is a bit of an aggressive grower under ideal conditions. It has a natural tendency to grow suckers from underground rhizomes. Cutting it down close to the ground is a technique usually used for plant rejuvenation.
If you want to rid of this plant you may want to prune it down to the ground and dig around it. Dig as much of it as possible. Due to the fact that it is such an old shrub, you will have to follow around the area as the rhizomes will extend to a larger area. You may want to wait for a season before planting anything in the hole so that you can eliminate any old suckers developing from the old roots. If you do not want to dig, you can try placing a dark plastic bag on top of it and covering it up with mulch. Pay close attention to avoid getting mulch on top on your Hostas.
For your health, and that of the soil around you, I strongly discourage you to use herbicides or pesticides. The product you mentioned is based on Glyphosate which works by inhibiting a specific enzyme that herbaceous plants need in order to grow. As it would affect any plants, you want to avoid spray drift going onto other plants, namely your Hostas.
Hope this information helps you with your plan.