Also exposure. Are the arbs to the south of the area you want to plant so you will have a towering wall of shade?

Growing zone is 6b in Michigan.

The arbs are west of the area to be planted, so the area would receive morning and some afternoon sun.

The species is called "techny arborvitae" and they should max out at 15' high by 8' wide. Yes, I'm trying to dress up the area and keep the weeds down. We want a grass-free backyard.

I’ve seen ’Techny’ grow to 18’x12’ and still growing.

tj

^^^ Yeah, conifers don't "max out" at a specific height. Or width. What you see reported is typically the 'mature' size (and 'maturity' can vary widely, both by definition and by species) but they will continue to grow throughout their lifetime.

Here are some ideas using viburnum & spirea:

Switch to Little Henry Sweetspire in place of the spirea if necessary.

or:

The photo makes it seem as if the root balls were placed at grade and soil was piled around them. This doesn't bode well for proper water absorption or protection from temperature fluctuations. Hopefully, any burlap or cage was removed prior to planting.

What is the length of the planting? The ultimate size of proposed material should be accounted for when planning. Viburnums and spirea can be sturdy, low maintenance, and lovely, but cultivars should be chosen that fit the spot. Shade will result in fewer blossoms. Can you assess the soil characteristics? Moist, sandy, clay? It's best to choose plants that are compatible with the conditions.

Are deer in your area? If so, deer resistant plants will be happier there.

Mt. Laurel. You don't want spirea, it's often invasive. The native ones like it wet and don't get as big. If you do viburnum, make it a native one.

This link talks about spirea:https://www.invasiveplantatlas.org/subject.html?sub=3076