Green foliage makes an ideal backdrop against which to display your flowers, mimicking how flowers would grow in the garden. It adds bulk to an arrangement, can tone down bright colours and is a useful framework for supporting your flower stems. I always add my foliage first and thread the flower stems among the foliage 'scaffold' when creating a display.
Evergreen foliage is ideal, as there is something to pick all year, but it can be hard in a residential garden to find space to grow shrubs and trees. If you are picking foliage all the time, you may leave your shrubs looking a bit bare, so go gently. I have a large, established Viburnum tinus, which we inherited with the garden, which can take a fair amount of cutting. I love the dark green, glossy foliage but I tend to wait until it produces creamy white flowers in the Autumn before using it. It has a wonderful fragrance to the flowers making it a welcome addition to bring into the house in the dark months of Winter. Other good evergreens include Rosemary, Box and any clippings from hedging plants. Experiment with the existing shrubs that you do have. There is a useful article by My Garden School about the best shrubs for cutting.
Great foliage plants
Herbs provide some of my favourite foliage to use during the Summer. Lemon balm, Borage and Oregano will all keep producing new foliage if you cut them. Once they are looking tired in mid-summer, you can cut the foliage back to the base and they will sprout up again giving you new, fresh foliage to use.
These include plant material, other than foliage, which provide an interesting backdrop to your flowers. Plants to try include the unopened flower buds of annuals such as Helianthus 'Vanilla Ice', Scabious or Cosmos, which all look interesting in bud form. You can try green flowers like Dianthus barbatus 'Green Trick' with its fuzzy, lime-green pom poms, the tiny acid-green flowers of Alchemillla mollis, Euphorbia oblongata, Bupluerum, Fennel or Dill or other annuals with tiny flowers such as Ammi majus or Ammi visigna. Some flowers, like Cosmos, come complete with their own foliage which you can leave on the stem (provided it is above the water line in the vase - if you leave on foliage below this, the water will quickly spoil, see Conditioning.)
Foraged Wild Carrot or Cow parsley are excellent fillers and abundant in the hedgerows. I've grown the grass Panicum elegans 'Frosted Explosion' for the first time this year and I will definitely grow it again. It adds a lovely airy feel to displays. There are lots of interesting grasses you could try such as Briza maxima (quaking grass) and Agrostis nebulosa.
For further inspiration and pictures of all the plants mentioned in this post, visit the Homeflower Pinterest page.