Planting airy perennials with grasses for the perfect hazy late summer garden.

As July ticks into August, the perennial garden begins to reach its climax. Notably, it is time when the so-called “warm-” or “late-season” perennial grasses start to hit their stride.

Late season grasses form the backbone of the modern continental style of perennial planting, giving sunny gardens height, structure, and colour which lasts right through the winter, even with deciduous grasses. Their fine foliage and airy flowers form the perfect counterpoint to more structural perennials, with their pink- and silver-touched new flowers (inflorescences) complimenting mid-season flowers and then turning cream and buff among the reds, yellows, and blues of autumn.

Their structural quality isn’t a subjective feature either. Their robust upright habits are a useful frame for perennials which are otherwise lax and sprawling to climb through or lean on in the wind.

But you don’t have to go for structural perennials. You can also emphasize the ephemeral, gauzy nature of grasses by going for airy perennials with a similar effect. The result can be a twitching haze of texture which tricks the eye into losing the boundaries of a space and making it feel bigger.

Sanguisorba tenuifolia

Sanguisorba Ruby Velvet

One plant we often get asked about at the nursery – typically with “What is that white-flowering grass?” – is Sanguisorba tenuifolia. This isn’t a grass – it’s a member of the rose family, Rosaceae – but it does have a similar quality, tall and airy, with an arching habit. They can grow to 1.8m or more and have pink or white flowers. We also have a couple of good selections of this species, such as Sanguisorba ‘Stand-up Comedian’ which is stronger growing, perhaps 2m or more on established plants, with white flowers from August to October.

There are a variety other Sanguisorba species also, varying on height, flowering time, and colour. From long wind-sock-like pink flowers of S. hakusanensis (early) or S. ‘Pink Brushes’ (late), to deep burgundy catkins or S. menziesii (early) and S. ‘Ruby Velvet’ (late).

Althea cannabina

Selinum wallichianum

Another wonderful tall plant is Althaea cannabina. This southern European plant is a relative of the native tree mallow (Malva arborea) but very delicate and refined by comparison. Very tall, to 2m in a season, with grey-green foliage and dozens of buds on branching wiry stems which open to pink flowers. Very drought tolerant, and impressively wind tolerant given their height. They are an upright plant in any open sunny site, but can lean a bit if not in full sun. Their most appealing feature for me is that out of endless buds, they only open a handful at a time. This massively extends the flowering period but, moreso, adds to that great feeling of potential energy a good garden should have where things are revealed gradually.

One of the most impressive perennials of August and September is the umbellifer (carrot relative, Apiaceae), Selinum wallichianum. This is a long-lived perennial plant with fine deeply divided foliage on strong purple stems which are topped with large plates of white flowers. An excellent pollinator plant and one which is tolerant of full sun if not too dry and partial shade, though best with some direct sun during the day. It likes a deep soil where it can establish a tap-root.

Patrinia scabiosifolia

Bupleurum falcatum

Patrinia scabiosifolia is another plant with airy plates of flowers, this time yellow and in the honeysuckle family, Caprifoliaceae. It has dark green scabious-like leaves and branching heads of fennel-like flowers. This year we also have an unusual selection with an even more delicate habit whose mane we don’t know so we’ve called it Chris after the staff member who found it.

For smaller plants and shorter heights, one of the many excellent Bupleurum species, Bupleurum falcatum is an exceptional plant with a 30cm x 30cm haze of tiny yellow flowers which last for weeks. Happy in any sunny site with not too rich free-draining soil. It is very good with grasses and other hazy plants like the ornamental oregano, Origanum laevigatum ‘Aromaticum’.

Dianthus carthusianorum

Nassella tenuissima

Dianthus carthusianorum is another small plant with an outsized presence, a wild carnation vastly simpler than the showy hybrid forms. It produces small red-pink flowers on tall stems and is excellent with ponytail grass (Nassella tenuissima) in a sunny dry border. It also combines well with one of the finest flowering perennials for growing with grasses, Verbena bonariensis. An effective plant in every sunny border it is tall and long-flowering but takes up no width. Dot a few plants around a border and let them self-seed to fill in the rest.

Grasses are an incredibly useful plant to grow for height and structure in an open sunny site. They can achieve the same effect as shrubs and even be used as screening without using woody plants or creating a canopy that can cast shade. They will support, visually and literally, the perennials around them and give great interest all the way through the winter. Deciduous grasses can be cut back to a few centimeters in spring. With grasses as the structural element in your planting, you can then use them to set off any number of flowering perennial plants.