Overgrown Caryopteris and Seedlings

I have three caryopteris plants along the back of a planting bed next to my house in southeast Wisconsin. They were planted in 2003 and grew much faster and bigger than everything else in the planting. Can I spray weed killer on the little plants coming up or will that kill the original plants too? Also, is it too late in the season to trim the big plants? I love the flowers in late summer, but I feel like the plants are out of control!

Caryopteris also known as blue mist spirea (Caryopteris x clandonensis) is rated hardy in zone 6 to 9 by some experts but mine, like yours is thriving in zone 5 and I have seen them perform well in parts of zone 4. Healthy vigorous plants will reseed readily. I invite friends to dig seedlings for their yards and then pull whatever is left. You can spot treat the seedlings with a total vegetation killer.  Whether using traditional or new environmentally friendly products read and follow label directions carefully. All these products are plant killers and can injure or kill your desirable plants as well as the weeds.

The best time of the year to prune them is late winter or early spring before growth begins. Prune the plants back to 4 to 6 inches above the ground. As the plants age you may decide to cut the plant back to 12 inches. Then remove some of the older stems back to an adjoining branch.

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