12 Perennials You Must Plant In September — Don’T Miss This 3-Week Window!

Go out to your beds this afternoon and push your fingers four inches down into the soil. Feel that? It is still holding summer. The air has cooled off, the nights have turned, but underground it is warm, and it will stay warm for another few weeks before it starts letting go. That gap between cool air and warm soil is the single most useful thing a gardener gets all year, and it lasts about three weeks in most of the country. Here is what happens in that gap. A plant put in the ground now does not have to make leaves. It does not have to make flowers. Nothing above the soil line is asking anything of it. So every bit of energy it has goes straight down into root production, and it does that in warm, moist soil, with no heat stress, no drought pressure, and no competition. Six to eight weeks of pure, uninterrupted root building before the ground locks up. That same plant put in the ground in April has to do three jobs at once with a root system the size of your fist. It has to push leaves, chase water in soil that is drying out a little more every week, and somehow build an anchor underneath it all. That is why spring planted perennials so often just sit there and sulk through their first summer while you wonder what you did wrong. You did not do anything wrong. You just showed up at the harder time of year. I am John Sam, and this is Seniors Gardening Advice. I have 12 perennials for you that belong in the ground this month, and I am counting them down from 12 to 1. Number one is the plant I see people get wrong more than any other, and it costs them years of flowers. Stay with me to the end for that one. Rather than repeating the same instructions 12 times over, let me hand you the general rules once, up front. These apply to almost everything on this list. Dig wide, not deep. A hole twice as wide as the root ball and exactly as deep. Roots travel sideways far more than they travel down, and a hole dug too deep settles over winter, which pulls your plant lower than you set it. Mix compost into the soil you dug out, roughly one part compost to three parts native soil, and use that mixture as your backfill. Do not fill the hole entirely with bagged potting mix. That creates a soft pocket the roots refuse to leave, and the plant sits in a bathtub every time it rains. Skip the fertilizer. Autumn is not the season for feeding. Nitrogen this late pushes soft green growth that winter will kill. Compost is enough. Water deeply once a week from the day you plant until the ground actually freezes, even after the leaves drop and it feels like nothing is happening. Underground, plenty is happening. A slow trickle from the hose for 15 minutes beats five minutes with a sprayer. Mulch two to three inches deep, but wait until after your first hard frost. Mulching too early keeps the soil warm and invites mice to move in for the winter, and keep it pulled back an inch or two from the crown of every plant. Last thing, label everything. In April, you will not remember what you planted in the back corner, and you will chop it down thinking it is a weed. I have done it. Twice. Perennial number 12. Crocus. Starting small on purpose, because the cheapest thing on this list gives you the biggest emotional return. Crocus corms cost pennies a piece, and they do something no other plant does. They push flowers up through frozen ground, sometimes through the last inch of snow, weeks before anything else in the garden has considered waking up. In a February that has gone on too long, that first purple cup opening in the grass does more for a person than any June border ever will. Plant the corms three to four inches deep, pointy end up, three to four inches apart, hardy through zones three to eight. The mistake nearly everyone makes is planting too few. Fifteen corms in a neat little circle looks like an accident. Plant them by the 50, scattered by hand, and planted where they land, and you get drifts instead of dots. And they multiply on their own. Each corm produces offsets underground, so a planting of 50 can fill twice its original footprint within four or five seasons without you lifting a finger. If you want the version that makes the neighbors slow down when they drive past, plant them directly into your lawn. Toss them across a section of grass, plant them where they fall, and then simply leave that patch un-mowed for about six weeks after the flowers finish, so the foliage can feed next year's bloom. What you get looks like a wild meadow arrived in your yard overnight. Perennial number 11. Ornamental Allium. Staying underground for a moment, because there is a second bulb that has to go in during this same window, or the whole year is lost. Alliums are the tall purple globes you have admired in photographs of English gardens, floating above everything else on bare stems in May. They need a long cold period in the soil to flower, which means the only chance you get to plant them is the one you are sitting in. Depth is two to three times the height of the bulb. A big one the size of a small apple sits about six inches down, a smaller one about three. Space them six to eight inches apart, full sun, and above all soil that drains. A bulb sitting in wet ground all winter turns to mush. The clever part is that they compound. Each bulb produces offsets, so 20 planted this autumn becomes 30, then 40, and the display grows without you buying another thing. One habit to plan around. As the flowers open, the strappy foliage at the base yellows and flops. It looks untidy right at the plant's best moment. So put your alliums behind something low that leafs out in spring. Hardy geranium, catmint, ornamental grass, anything that fills in and hides the mess. Do that now, and it looks deliberate later. Worth asking for by name at the nursery. Gladiator for big purple heads on three-foot stems, Mount Everest for clean white, and Millennium for shorter late summer flowers that bees smother completely. If this is the sort of practical detail you want more of, a subscribe costs you nothing and keeps these lists showing up for you. Perennial number 10, blue mistflower now, to something you will not find at the garden center, which is exactly why I want you to have it. Blue mistflower is a native perennial from the eastern and central states that opens in late August and keeps going deep into October in soft fuzzy clusters of a hazy blue violet that hovers over the foliage like smoke. That color is nearly impossible to find in autumn from anything else. Not lavender, not periwinkle, a genuine cloudy blue at the exact time of year when everything else has gone gold and rust. Its timing is the entire point. Bees and butterflies working in September and October are running out of options fast, and migrating monarchs passing through need fuel more than they need anything else. This plant serves them at the leanest moment of the season. It takes full sun to part shade, and, unusually, it likes moisture. Low spots, the ground near a downspout, a rain garden, anywhere that stays damp longer than the rest of the yard. Hardy in zones 5 through 10, growing 2 to 3 feet tall, spreading slowly by underground runners, and filling in over the years without taking over. Look for it at native plant nurseries or reputable online growers. It is worth the extra effort of tracking down. In my experience, the plant that makes a visitor pause and ask what they are looking at is never the rose or the hosta. It is always the one they have never seen before. Perennial number 9, turtle head. Since we are already in the damp corner of the garden, let me give you the plant that solves the problem spot most people have written off entirely. Every garden has one. The low patch that stays soggy for two days after rain. The shaded strip along the north side of the house. The bottom of a slope where water collects and sits. You have tried three things there and killed all three. Turtle head is what belongs there. It is a native perennial, named for flowers that look exactly like a turtle poking its head out of its shell, blooming from August into October on sturdy upright stems 2 to 3 feet tall in rose, pink, or white. The blooms are unusual enough that first time visitors do a double take. Part shade and consistently moist, humus rich soil is not what it tolerates. It is what it wants. Work compost in generously when you plant. Water it faithfully through the first season and after that it asks for almost nothing. No staking, no deadheading, no dividing for years. Hardy through zones 3 to 8. This is also the plant on the list that suffers most from spring planting. It needs a full season to build the root mass that supports heavy flowering. Put it in this month and it has all autumn to do that work quietly. Put it in next April and it spends its first summer just trying to catch up and the flowers show it. Perennial number eight. New England aster sticking with late bloomers because this next one is already flowering as you watch this. Walk into any nursery this week and the asters are covered in color. That is not a coincidence. It is what they do. New England aster blooms from late August through October and sometimes into November in mild areas right through the leanest stretch of the whole gardening year. The flowers are simple daisies, deep purple, hot pink, or white with a bright gold eye carried on upright stems that can reach three to six feet in a good year. Full sun, average soil that drains. Hardy in zones 4 through 8. There is one quirk that catches people out. By July the plants get tall and bare at the ankles before they ever flower and it looks alarming. The fix is easy. Cut every stem back by half in late June. Bloom arrives a week or two later than it would have but on a shorter bushier plant carrying far more flowers. If you would rather not think about it at all, buy the compact varieties like purple dome and skip the whole business. Divide the clumps every three or four years in spring to keep them vigorous and because it is native across most of the east, it feeds the same insects and migrating butterflies that have depended on it for thousands of years. That is a lot of value from one plant in one difficult month. Perennial number seven, blue star. From a plant that gives you one long season to one that quietly earns its keep three separate times a year. Blue star sold under the botanical name Amsonia is walked past constantly at nurseries because it is rarely in bloom when people shop. Their loss. In late spring it opens clusters of small pale periwinkle stars in a soft clear blue that is genuinely hard to source anywhere else in the perennial world. Through summer the flowers fade and the fine feathery foliage takes over forming a neat rounded mound that holds its shape through July heat without flopping open in the middle the way so many perennials do. Then October arrives and the whole plant turns to gold, butterscotch, amber almost glowing when the low afternoon sun comes through it. It is one of the finest pieces of autumn color available from a perennial and it comes from a plant nobody bought for autumn color. Full sun to light shade average well-drained soil hardy in zones four through nine drought tolerant once settled and largely ignored by deer. One firm rule do not divide it and do not move it. Blue star sulks for years after being disturbed choose the spot carefully plant it once and leave it alone for the next 20 years. If you know someone who keeps saying their autumn garden looks tired send them this video this is the plant that fixes it. Perennial number six oriental poppy now for one that genuinely cannot be planted properly at any other time of year. Oriental poppies bloom in late May and early June those enormous crepe paper cups in scarlet coral and salmon with a black blotch at the base and then in July they do something startling they yellow collapse and plant has not died it has simply gone to sleep for the summer the way spring bulbs do which is exactly why autumn is its moment right about now fresh rosettes of new leaves push up from that dormant root and that is the plant telling you it is awake and ready to be planted or moved nurseries ship bare roots for this reason set the crown about three inches below the surface full sun and drainage that is genuinely good a poppy will forgive drought poor soil and neglect it will not forgive a winter spent in heavy wet clay hardy in zones three through seven and it dislikes hot humid summers so gardeners in the deep south should pass on this one plan for the summer disappearing act while you are planting put something beside it that fills the gap in July catmint baby's breath or a hardy geranium works beautifully then the hole it leaves behind never shows perennial number five bearded iris sticking with plants that demand a late summer planting date here is the one your grandmother almost certainly grew bearded iris rhizomes go in from late July through September and September is the tail end of that window in most regions they need warm soil to root before winter which is why nurseries ship them now and not in April the technique is unlike anything else on this list so pay attention to this part the rhizome that thick fleshy root that looks a bit like a piece of ginger must sit at the surface with its top actually visible exposed to the sun dig a shallow hole build a small ridge of soil in the middle drape the roots down either side of the ridge and firm it in with the top of the rhizome showing bury it and it will rot or it will live and never flower full sun at least six hours sharp drainage and do not mulch over the rhizomes whatever you do elsewhere in the garden they want to bake space them a foot to 18 inches apart with the fan of leaves pointing in the direction you want the clump to travel trim those leaves back to about six inches so the wind cannot rock the plant loose before it roots divide every three or four years again in late summer crowded iris clumps flower less every year until they quit altogether and the fix is nothing more than lifting splitting and replanting the healthy outer pieces perennial number four hellebore the next one gives you flowers at a time of year when you have no right to expect any hellebore also sold as lenten rose blooms in february and march occasionally january in mild areas while frost is still on the ground and the rest of the garden is sound asleep nodding cupped flowers in white blush plum deep wine and shades so dark they read as black carried above evergreen leathery leaves that hold on all winter long planting it this month gives it four or five months of root development before it has to perform by the time february comes around it is ready it is one of the few flowering perennials that prefers shade so put it under deciduous trees along the north side of the house or anywhere with dappled light rich soil with plenty of compost worked in good drainage and set the crown level with the surrounding soil rather than below it a hellebore planted too deep produces leaves and withholds flowers two practical notes the sap can irritate sensitive skin and the plant is toxic if eaten so wear gloves when you handle it and think about placement if grandchildren or dogs are regularly in that bed and in late winter cut away last year's tired leaves just as the flower buds appear that single 15-minute job transforms how the plant looks in bloom hardy in zones four through nine living for decades needing no division ever a mature clump can carry 50 flowers at once perennial number three true lilies we are into the top three and this next one is the plant people spend the most money on and treat the worst i mean true lilies the genus lillium not day lilies asiatics that open in june orientals with that heavy perfume in august and the orion pet crosses that grow chest high and carry a dozen blooms per stem lily bulbs are unlike tulips or daffodils in one crucial way they have no papery skin just overlapping fleshy scales which means they dry out on a shelf and never fully recover buy them from a grower who ships them fresh in autumn and get them into the ground within a few days of arrival that alone puts you ahead of most people plant them deep six to eight inches to the base of the bulb roughly three times the bulb's height that depth matters more than it does for other bulbs because many lilies produce a second set of roots off the stem above the bulb and those stem roots are what keep a four-foot flower stalk standing upright in july wind full sun on the flowers cool shaded soil at the feet plant them among lower perennials that shade the ground and mulch sharp drainage is non-negotiable then leave the foliage alone after flowering that green stem is feeding next year's bloom so let it yellow completely before you cut it hardy in zones four through eight and if you garden in the northeast keep an eye out for the bright red lily leaf beetle hand picking in early spring is the simplest control there is perennial number two false indigo second to last and this is the plant i would put in the ground if someone told me i could only choose one thing to outlive me false indigo sold as baptisia sends down a tap root that can reach several feet into the earth that root is why it survives drought that flattens everything around it why deer leave it alone and why it can sit in the same spot for 30 years without ever being divided fed or fussed over in late spring it throws up spires of pea-shaped flowers the classic species in indigo blue newer varieties in butter yellow chocolate and smoky purple when the flowers finish the blue green foliage forms a shrub-sized mound three to four feet across that stays clean and upright until frost then black seed pods appear that rattle in the wind and look wonderful in a dried arrangement full sun average soil hardy through zones three to nine one thing you must know before you buy and it is the reason people give up on this plant baptisia is slow the first year it sleeps the second year it creeps the third year it leaps that is not a saying invented to make you feel better it is exactly what happens while all that energy goes into the tap root give it three years and it becomes the most self-sufficient plant you own because of that tap root choose the site once and mean it a mature baptisia does not want to be moved and honestly at a certain point in life neither do we perennial number one peony which brings us to the plant i promised you at the start and to the mistake that costs more gardeners more flowers than anything else i know peonies must go in during autumn they are sold bare root in september and october for a reason planted now the root spends the next six weeks pushing fine feeder roots out into cooling soil so that when spring arrives the plant is already established and simply gets on with growing a spring planted peony spends two or three seasons catching up to where an autumn planted one started now the part that matters most look at the bare root and you will see pinkish red buds on the crown those are the eyes and they must sit no more than one to two inches below the soil surface one to two in warm regions closer to one in cold regions closer to two and never deeper plant a peony too deep and it will not die that is the cruelty of it it will come up every single spring with beautiful healthy foliage and give you nothing but leaves for years while you wonder what is wrong with your soil nothing is wrong with your soil the crown is buried and the plant needs to feel winter cold at that shallow depth to set flower buds if you have a peony that has not bloomed in three years dig it up this month look at how deep the eyes are sitting and reset it people write to me about that one fix more than any other the rest is simple full sun six hours minimum soil that drains compost worked into the backfill no fertilizer at planting space them three feet apart because they will fill that space hardy in zones three through eight and then they simply stay there are peonies still flowering on old farmsteads where the house burned down 80 years ago you plant this thing once correctly at the right depth in the right month and it outlives you if fragrance matters to you and it should ask for the old double whites and pinks festiva maxima duchesse de no more sarah bernhardt those are the ones that fill a whole room from a single stem in a jar on the kitchen table 12 plants crocus allium blue mist flower turtlehead new england aster blue star oriental poppy bearded iris hellebore lilies false indigo and peony you do not need all 12 choose two or three that suit the spots you already have trouble with get them in while that soil is still warm water them until the ground freezes and let autumn do the work for you underground where you cannot see it the difference between a garden that disappoints and one that quietly gets better every year is almost never talent and it is almost never money it is timing and the timing is sitting right outside your back door for about three more weeks if this was useful a like helps more people find it and subscribing puts the next one in front of you now go put something in the ground that will still be there long after you have forgotten planting it