The room you need comes down to one number: the mature width of the variety you buy. Good arborvitae spacing matches the gap between trunks to that width, so the plants meet and fill in without fighting each other for the same patch of ground. A row that looks bare at planting time is doing it right.
A tightly packed row tricks you at first. It looks full and lush for the first two years, then it starts to thin and brown along the inside where neighbors lean into each other. Crowded plants shade each other out, and the shaded branches drop their needles. You end up with green tips and a hollow, woody middle. From the road it reads as a patchy fence, not the solid green wall you paid for. The thinning always shows up worst on the side that gets the least sun, since those branches were already working with the smallest light budget.
That brown does not come back. Arborvitae will not push new growth from bare old wood, so the gaps you create by crowding are there for good. This is why how far apart to plant arborvitae matters more than almost any other choice you make on planting day. Space them right once and you save yourself a permanent eyesore.
The rule is simple. Space each plant by its mature width and the row knits into one solid wall as the plants grow. The branches touch, weave together, and block the view without any single plant getting choked. Give them less room than that and you force the same bare interior that the no-regrowth habit then makes permanent. Some people nudge the gap a touch tighter, by maybe six inches, to close a screen a season faster. That is fine. Going much past that point is where the trouble starts, so treat the mature width as your floor, not a number to beat.
The right number changes a lot by variety. Emerald Green stays narrow and wants only about 3 to 4 feet between plants. Green Giant grows huge, so good arborvitae hedge spacing for it sits around 5 to 6 feet apart for a screen. That sounds far when the pots are small, but Green Giant reaches a mature width of 12 to 20 feet, and a screen only needs the crowns to touch, not the trunks.
A single specimen plant needs even more breathing room than a hedge. You are not trying to merge it with a neighbor, so give it the full mature width on every side and let it show its natural shape. A Green Giant standing alone wants a clear circle of 12 feet or more around it, which surprises most people who buy it as a cute three-foot pot.
Before you buy anything, grab a tape measure and check the bed width against the mature spread. Measure how long the run is, then divide by the spacing for your chosen variety to get your plant count. A 40-foot screen of Emerald Green at 4 feet apart needs about ten plants, while the same run in Green Giant at 6 feet needs only seven. If the bed is only 3 feet deep, a Green Giant will outgrow it and crowd the path, so pick Emerald Green instead. Matching the plant to the space beats squeezing the wrong plant into a tight bed every time.
Read the full article: Arborvitae Shrub: Complete Growing Guide
