Peter Dowdall: Here's what to do in your garden in September
Late summer-flowering perennials, like this Verbena bonariensis, are still giving of their best and will root easily from cutting taken now. File picture
September has a way of catching us off guard in the garden. The calendar says autumn is here, yet the borders are still colourful, the grass is green, and there’s warmth left in the sun. It’s a lovely time to simply enjoy what’s still in flower, but it’s also a time of opportunity.
We gardeners are nothing if not optimistic, and this is the month to plant for the future, to look to next spring and early summer while now, everything else is winding down. If you take advantage of it now and work a season or two ahead, your garden will repay you many times over.
Spring colour isn’t something you organise in late winter, because by then you can already be too late for the best results. September is perfect for sowing hardy annuals such as nigella, cornflower, and calendula. Sown now, they germinate quickly in the still-warm soil, then quietly overwinter as small plants before exploding into bloom much earlier than their spring-sown counterparts. The same goes for biennials, wallflowers, honesty, sweet williams, and foxgloves, which need a long run-up to give their best display. In many cases, if you wait until spring to sow or plant them, you won't see their true potential until the following year.
This month is also the time to divide and replant spring-flowering perennials. Clumps of primroses, pulmonaria, and hardy geraniums can be lifted, split, and replanted while the soil is still workable and warm enough for roots to re-establish before winter. Doing this now, rather than in spring, gives you healthier plants and better flowers, quicker next year.
If you have gaps in the garden, even just patches of bare soil between perennials, think about slipping in some evergreen structure. Small shrubs like hebe, dwarf pittosporum, box or its alternatives, such as Ilex crenata give you something to look at when the herbaceous plants die back, and they anchor the whole border visually, providing consistency throughout the year.
Planting them now means they will have developed a better root system by next summer than those planted next year. It’s also a chance to add winter-flowering shrubs such as viburnum or mahonia, which bring not only form but also fragrance and food for early pollinators.
While September is a great planting month, it’s also a fine time for propagation. Many tender perennials and sub-shrubs respond well to cuttings now, giving you free plants for next year. Salvias, pelargoniums, penstemons, fuchsias, and verbena bonariensis can all be propagated easily from semi-ripe cuttings taken this month. Snip non-flowering shoots about 7–10cm long, remove the lower leaves, and pot them up in a free-draining mix. Keep them in a bright, frost-free place over winter and you’ll have sturdy young plants ready to plant out in spring. Lavender, rosemary, and other woody herbs can also be started this way, a cost-effective way to fill out a border or refresh an ageing plant.
The September sun is lower in the sky, casting longer shadows and a warm glow that can make seed heads, ornamental grasses, and even the simplest foliage look magical. Ornamental grasses like miscanthus, molinia, and panicum are at their peak, catching the light and moving beautifully in the breeze. The dark outlines of echinacea, the airy sprays of verbena bonariensis, and the feathery foliage of fennel all take on a new richness in this softer light. If you don’t yet have plants for autumn interest, now is the time to add them so you can enjoy this display in years to come.
September is a unique time in the garden that feels different from any other time of year. It’s busy, but not frantic. There’s space to think, to plan, and to take deliberate action without the pressure of spring’s mad rush. It’s a month of calm urgency, calm because the heat and high demands of summer have eased, urgent because the window for planting and propagation before winter is brief. Soon enough, the soil will be cold and wet, and the evenings will close in. The one thing we all know is that the calendar stops for no one, and if you miss the chance, it’s gone until next year.
So make the most of this moment. Sow your hardy annuals, plant your biennials, divide your perennials, slip in a few evergreens, and take those cuttings. Every job you do now is an investment, quietly working away beneath the soil or in a sheltered corner, ready to reward you when the garden wakes again. September gardening is all about that mixture of present enjoyment and future promise.
If you’re in Cork this weekend, why not join me at Gairdin Garden Centre at Bandon Co-Op this Saturday, September 6, as we celebrate its exciting relaunch?
