A spreading plant keeps widening — by runner, by rooting where branches touch, or just by growing outward year after year. Used deliberately, it is the cheapest way to cover ground, because a few plants eventually do the work of many.
Used carelessly it is the most common planting mistake there is. A spreader planted at the spacing that looks right on install day is a spreader growing into its neighbors by year four, and the fix is always removal — either of it or of what it crowded.
So the number that matters is mature width, and it has to be believed. Half the mature width from any hard edge, half the combined widths between two plants. A bed spaced that way looks sparse the first year, correct in year three, and like the drawing by year five.
Worth separating two things the same word covers. A plant that spreads by slowly widening its own crown is predictable and stays one plant. A plant that spreads by runner or by self-seeding travels, and traveling is a different commitment — near a lawn, a fence line, a ditch or open ground, that is the question to ask before it goes in, not after.
