Most gardeners think of spring as perennial planting season. And while spring is certainly a fine time to plant, there is a compelling argument that late summer, right now in August and into September, is actually the best time of the entire year to add perennials to your Ontario garden. Not just a good time. The best time.
Here is why. Soil temperatures in late summer are at their warmest of the entire year, which means perennial roots establish quickly and enthusiastically in the weeks after planting. Air temperatures are beginning to moderate from the peak of summer heat, reducing transplant stress dramatically compared to planting in the intense heat of June and July. Ontario's reliable September rainfall does much of the watering work for you. And the long cool stretch of autumn ahead gives newly planted perennials eight to ten weeks of comfortable root building time before the ground freezes, setting them up to emerge in spring as established, vigorous plants rather than fresh transplants.
There is also a practical financial advantage worth mentioning. Late summer and early fall are when garden centres across Ontario begin discounting their perennial stock to clear space for fall inventory. The same plant that cost full price in May is often available at a significant discount in August and September and it will perform just as beautifully in your garden next spring regardless of when you paid for it.
Here are the five best perennials to plant right now for years of colour in a Zone 6a Ontario garden.
1. Coneflower (Echinacea purpurea)
Bloom time: July through September
Mature height: 60 to 90 centimetres
Sun requirement: Full sun to partial shade
Zone 6a hardiness: Fully hardy, reliably perennial
Coneflower is one of the most versatile, reliable, and ecologically valuable perennials available to Ontario gardeners and a plant that belongs in virtually every garden in the GTA. Its large daisy-like blooms with distinctive raised centres come in the classic purple-pink of the native species as well as an expanding range of colours including white, orange, yellow, and deep red in named varieties. It blooms from midsummer through early fall, attracts bees and butterflies prolifically through its long flowering season, and provides valuable seed heads for goldfinches and other seed-eating birds through winter if left standing.
Planted now in late summer, coneflower establishes quickly in warm soil and will emerge as a noticeably stronger and more floriferous plant next July than a fresh spring transplant would be. It is drought tolerant once established, largely pest and disease resistant, and requires almost no maintenance beyond cutting back old stems in spring.
Pro Tip: Plant coneflower in groups of three or more for the most impactful display and the greatest pollinator attraction value.
2. Black Eyed Susan (Rudbeckia fulgida)
Bloom time: July through October
Mature height: 45 to 90 centimetres depending on variety
Sun requirement: Full sun
Zone 6a hardiness: Fully hardy, self seeds freely
Black eyed Susans are one of the most cheerful and reliably productive perennials in the Ontario garden, producing their golden yellow daisy flowers with dark chocolate centres in abundance from midsummer right through to the first fall frosts. Rudbeckia fulgida varieties like Goldsturm are particularly long-lived and reliably perennial in Zone 6a, forming expanding clumps that can be divided every three to four years to rejuvenate the planting and create new plants for other areas of the garden.
Planted in late summer, black eyed Susans establish their root systems quickly in warm soil and emerge strongly the following spring. They are exceptionally tolerant of Ontario's clay soils, handle summer heat and drought with equanimity once established, and require virtually no intervention beyond an occasional division to keep them from crowding neighbouring plants.
Pro Tip: Leave seed heads standing through winter. They feed birds and provide structural winter interest in an otherwise bare garden bed.
3. Autumn Sedum (Hylotelephium spectabile)
Bloom time: August through October
Mature height: 30 to 60 centimetres
Sun requirement: Full sun
Zone 6a hardiness: Fully hardy, extremely drought tolerant
Autumn sedum is the unsung hero of the late summer and fall Ontario garden. While most perennials are winding down by September, sedum is just getting started. Its flat-topped flower heads in shades of pink, rose, and deep burgundy open in late August and September and provide some of the most valuable late-season nectar available to bees and butterflies preparing for winter in the GTA. Its succulent blue-green foliage is attractive from spring through to its blooming period and the dried flower heads remain ornamental and structurally interesting well into winter.
Autumn sedum is among the most drought tolerant perennials available for Zone 6a Ontario gardens, thriving in hot, dry, well-draining conditions where many other plants struggle. It is also one of the most foolproof perennials to plant in late summer since its succulent nature makes it particularly resilient to the establishment process.
Pro Tip: Autumn Joy and Matrona are particularly reliable varieties for Zone 6a and are widely available at Ontario garden centres through late summer.
4. New England Aster (Symphyotrichum novae-angliae)
Bloom time: September through October
Mature height: 60 to 120 centimetres depending on variety
Sun requirement: Full sun
Zone 6a hardiness: Fully hardy, native to Ontario
New England aster is the plant that keeps your garden beautiful when almost everything else has called it a season. Blooming in rich shades of purple, violet, pink, and white from September right through to the first hard frosts of October, it provides some of the most valuable late-season nectar in the Ontario garden for monarch butterflies, bumblebees, and native bees preparing for winter. As a native Ontario plant it has evolved alongside local pollinators and provides food sources that introduced ornamental plants simply cannot replicate.
Planted now in late summer, New England aster establishes quickly and will bloom within weeks of planting in its first season, giving you immediate colour while also settling in for years of reliable autumn performance. Pinch plants back by one third in late June next year to encourage bushier, more compact growth and prevent the legginess that can develop in taller varieties.
Pro Tip: New England aster combines beautifully with ornamental grasses and autumn sedum for a late-season display that peaks when most other gardens have already faded.5. Catmint (Nepeta racemosa)
5. Catmint (Nepeta racemosa)
Bloom time: May through September with deadheading
Mature height: 30 to 60 centimetres
Sun requirement: Full sun to light shade
Zone 6a hardiness: Fully hardy, very drought tolerant
Catmint is one of the most useful and versatile perennials available for Zone 6a Ontario gardens, producing a continuous froth of lavender-blue flowers from late spring through summer and reblooming generously after being cut back. It is exceptionally drought tolerant once established, largely deer and rabbit resistant, beloved by bees and butterflies, and works beautifully as a border edging plant, a ground cover on sunny slopes, or a soft companion to roses and other structural plants.
Cut catmint back by two thirds after its first main flush of bloom in early summer and it will produce a fresh new mound of silver-green foliage and a second generous flush of flowers within three to four weeks. Planted now in late summer it will establish quickly and emerge next May ready to bloom immediately, giving you a significantly longer first season display than a spring planted transplant would.
Pro Tip: Walker's Low is the most widely available and reliably performing catmint variety for Zone 6a Ontario and is an excellent starting point for gardeners new to this plant.
How to Give Late Summer Planted Perennials the Best Start
The perennials you plant now have a compressed timeline for root establishment before the ground freezes in November. Giving them the best possible soil conditions at planting time accelerates that establishment process and significantly improves their chances of emerging strongly next spring.
Remove any existing weeds or spent annuals from the planting area completely. Incorporate a generous layer of compost into the top 20 centimetres of soil along with a slow release granular fertilizer formulated for perennials. Water newly planted perennials deeply and consistently through their first four to six weeks in the ground even as September temperatures moderate. Apply a 7 to 10 centimetre layer of mulch around newly planted perennials before the ground freezes in November to insulate roots from the freeze-thaw cycles that can heave newly planted perennials out of the ground over winter.
Stop by Lakeside Garden Gallery and let our team help you choose the right perennials for your specific space, sun exposure, and soil conditions. Late summer is when our perennial selection is at its most value-priced and our team can help you build a planting plan that delivers colour across every season for years to come. 🌸🍂🌿