Filler plants for flower border
It’s easy to overlook filler plants for the flower border or flower bouquet. In any garden there are star plants, they demand our attention, through their striking colour, height or shape. They stand out from the crowd and their star status is obvious. But not every plant in the border can be a star. If there are only stars, each one can become lost in the noise of plants demanding our attention. This is where filler plants come into their own!
Often understated, but oh-so-essential for a beautiful border. I particularly like filler plants that are light and airy, that hold their blooms above high above the foliage, and if they have feathery foliage too, I like them even more. Just because they are filler plants, doesn’t mean that they aren’t beautiful or striking, but they are usually more subtle or muted, with a multitude of tiny flowers rather than fewer large ones.
Repeat planting of your chosen filler plants across your garden bed will add a sense of cohesion and help to draw the eye across the planting. Here is a selection of my favourite light and airy plants to use in a supporting role in the garden.
Thalictrum
The leaves of Thalictrum plants remind me of Aquilegia, but the flowers are very different. Thalictrum produce a pom pom of fine fluffy flowers on tall stems and come in shades from purple to pink through to white and yellow. Most varieties are happy in partial and light shade. I spotted Thalictrum ‘Black Stockings’ in the Forest Bathing Garden at RHS Chelsea Flower Show.
These lovely plants are available in a variety of heights, from the 160cm tall Thalictrum Rochebruneanum (which I’ve recently bought to grow towards the back of a border) to the diminutive Thalictrum kiusianum at 10-15cm tall. There are taller varieties, but I’m less inclined to think of those as filler plants and more as statement plants in their own right.
Verbena bonariensis
Instantly recognisable, Verbena bonariensis has become much-used by garden designers to add height and volume to borders. And it is popular for good reason, it grows easily from seed and attracts pollinators to the garden. Weed out unwanted seedlings in the spring. To avoid too many self-sown plants, do not cold compost the plants with seed heads, save them for a hot compost pile or add to the green waste bin.
Knautia macedonica
This is one of the plants that I am asked about most frequently in my garden. It forms an untidy mound of foliage, above which strong burgundy pincushion flowers of Knautia macedonica appear on long stems. The flowers do not last very long, but with regular deadheading, it will continue to produce flowers from May until well into autumn. Height 80cms and spread up to 50cms, although the lax foliage often gives the appearance of a much wider spread. If you prefer a paler pink, try Knautia macedonica ‘Melton pastels’ for pink, rose and crimson flowers.
Gypsophila paniculata
Even if you aren’t familiar with this plant in the garden, you will probably recognise it from floral bouquets. It is commonly known as Baby’s Breathe. And used by florists to add volume and give a romantic look to a bunch of flowers or bridal bouquet. And it can be a great filler plant in the flower border. Although the white flower variety is most common, there are varieties with pink and mauve flowers too. Last year I grew a pink Gysohphila paniculata, but it didn’t survive the very wet winter. It prefers well drained soil and to grow in full sun.
Anthriscus sylvestris ‘Ravenswing’
This variety of cow parsley has very dark stems and leaves and offers a great foil to paler leaved plants, and a mass of frothy white flowers. Anthriscus sylvestris ‘Ravenswing’ do a good job of filling a space without overpowering it and enable other plants to grow up and through their stems. Similar in habit, Anthriscus sylvestris ‘Golden Fleece’ has light green leaves to add a splash of brightness to an area. It is easy to grow from seed and will readily self-sow in the garden. However, it is easy to remove any unwanted plants when they are young.
Trachelium caeruleum
Throatwort is a tender perennial plant, often treated as an annual. It grows to around 90cms high and produces domed clusters of flowers. Purple, mauve or white are the most usual flower colours, but a deep maroon variety is available, Trachelium caeruleum ‘Black Knight’.
Filipendula ulmaria
Another of the good filler plants for the flower border is Meadowsweet. It is a relatively unfussy perennial plant that grows to around 90-100cms tall and up to 100cms spread. It grows vigorously, so may not be suitable for smaller gardens. The flowers are either pink or white and can be used for culinary purposes (check for potential allergy issues). They add a sweet, honey, almond, hay flavour to food. Filipendula ulmaria prefer moist by free-draining or even poorly draining soil and will grow in all aspects.
Valeriana officinalis
I spotted common valerian in a border on a garden visit to Llysdinam Gardens (Powys) in May and instantly liked it. Actually, I could smell it before I saw it. I detected the strong scent from two or three metres away.
The pinnate leaves work as a space filler and the flower stems rise up to 150cms. Plants are propagated by seed or basal cuttings in spring, or by division in spring or autumn. Not to be mistaken with the invasive red valerian, which is actually Centranthus ruber.
Add an Edible
- Parsley. I grow a lot of parsley, not only for using in the kitchen, but to provide ground cover and act as a filler. The yellow-green flowers umbels are held aloft from the mound of parsley leaves. The flowers attract pollinators and pest’s predators to the garden.
- Herb fennel. It makes excellent filler plant, although it can grow quite tall as it matures. The leaves are fine and feathery and the bronze variety provides a good contract to many garden stars. I like to grow bronze fennel between dahlias and peonies to provide contrast between the strong shapes of the leaves and big flower heads.
Thrifty tip
Don’t be in a rush to clear away seedlings too quickly in the spring. I like to wait until the two or four true leaves stage, so I can check for self-sown seedlings of filler plants. Transplanted into pots and grown on, before planting them out for more even more flowers next year.
More information
The article, Filler plants for flower border, was originally written for Amateur Gardening magazine in early summer 2024. I have updated some of the specific varieties and images and may have added further plants to the list. Additionally, I’ve included to help you find the suggested plants.
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