Three plants answer that question most of the time. Catmint, Russian sage and salvia all send up soft spikes in the same washed out violet, and from across a yard you read them as lavender. Purdue Extension lists all three as separate purple perennials, not lavender at all. So the lavender look alike flowers in your neighbor's bed are likely one of those three, and the tells are easy once you know where to look.
"That's your lavender, isn't it?" my neighbor asked, pointing at the front edge of the bed. I planted Walker's Low catmint along that edge years ago, and it had been in bloom since May. My lavender sat two rows behind it, tighter and stiffer, and she had walked right past it on her way in.
Crouch down and you can split the two apart in seconds. Lavender packs its flowers into a tight plug at the tip of a bare stem, and the leaves feel narrow, gray and stiff between your fingers. Catmint versus lavender comes down to that stem. Catmint scatters small flowers loose along the whole upper stem, the leaves are soft and gray green, and the plant settles into a low sprawling mound instead of a tidy ball.
Russian sage pulls the same trick on you from farther off. The stems are whitish, the foliage is silver and finely cut, and the flowers float in an open branched cloud rather than one solid spike. Salvia goes the other way. You get dense upright spikes over plain green leaves, and the color often runs deeper and truer purple than the gray toned bloom you see on lavender.
Plant habit gives you the answer from a distance. Lavender grows as a small woody shrub and keeps its rounded outline all season long. Catmint stays soft right down to the ground, and by midsummer it flops over your path edge. Russian sage stands tall and see through, so you can look straight past the flowers to whatever grows behind it. Sort them by shape first and the color matters much less.
You have more than three suspects. Anise hyssop, spike speedwell and betony all carry upright purple flower spikes, and any of them can pass for lavender in a photo on your phone. Across seven popular purple flower roundups, catmint and lavender each show up on six of the seven lists. That is a fair sign of how often you find the two filed together. One more plant copies the same spike and brings real trouble with it.
Purple loosestrife (Lythrum salicaria) came over from Eurasia, spread in part as a garden ornamental, and now crowds out native species where it takes hold. It forms thick stands of straight stems topped with showy purple bloom, and it does that in wet ground where nobody planted it. From your car window it looks like a lavender field. Up close you are looking at a problem plant, not a perennial you want.
A purple spike 4 to 7 feet (1.2 to 2.1 m) tall standing in wet soil or shallow water, with 5 to 7 petals on each small flower, blooming July through September, is likely purple loosestrife.
That height alone rules out the garden three. Lavender, catmint and salvia all want dry, sunny ground, and none of them stand 4 to 7 feet tall in a marsh. If your plant passes that check, leave it where it is and look up your state noxious weed list before you dig one into a bed. Buy a labeled plant from a nursery instead, since catmint and salvia give you the same look with none of the risk.
Read the full article: 10 Purple Flowers Worth Planting
