Blue is the ultimate mixer, it works with all other colours and is the magic ingredient to add to almost any planting.

It can link two colours together that would normally scream at one another: Purple and orange become exciting and adventurous in the presence of blue and even yellow and pink sit in harmony.

Blue is a good colour in English gardens because it becomes more visible in low light: a particular attribute on dull, cloudy days; something we have not been lacking through late spring and early summer this year. Blue shades are at their best in morning and evening, times when we are often admiring the garden.

Blue is a popular colour, we like blue in our homes, wardrobes and in our gardens, however, true blue is not a colour to be found amongst some of our favourite flowers. There are no truly blue roses, lilies, tulips, dahlias or chrysanthemums. However, some flowers are associated with the colour blue and this guarantees their popularity. Delphiniums are the most obvious example. Statuesque border plants producing dramatic flower spikes they may be, but it is usually the colour that grabs our attention. Despite their popularity, delphiniums are not the easiest of plants to grow. They like a fertile soil, rich in organic matter, and dislike over-wet conditions in winter. The emerging shoots need protection from slugs and snails; a mulch of sharp grit over the soil surface around the plants helps to deter these pests. Only the varieties of Delphinium elatum are reliably perennial. These are best sourced from a specialist grower. Most delphiniums are seed raised and are best replaced every two to three years. After flowering all delphiniums should be cut right down to the ground. Feed with Vitax Q4, mulch with compost and water thoroughly and they should produce another crop of flowers in late summer.

Herbaceous geraniums are such versatile plants, thriving on any soil, particularly chalk, and flowering freely in sun or partial shade. They are obvious planting partners for roses, filling space at the front of the border and successfully concealing the lower part of mature rose stems that can be rather bare and unsightly. Most make soft, rounded mounds of attractively-cut foliage and produce a succession of flowers over many weeks. Geranium Buxton's Variety' used to be one of the most sought-after varieties with its white-eyed, blue flowers on a low mound of foliage. Geranium Jolly Bee' is a superb alternative with large, open saucer-shaped blooms and excellent foliage. It never looks its best in a pot when sold but do not be deceived, it is an excellent plant in the border. Geranium Orion' is taller and more upright with a long flowering period. It is a good alternative to the ever popular Johnson's Blue'. If geraniums are cut back after flowering, they quickly produce a fresh mound of leaves and often further flowers in late summer.

The perennial salvias are useful for late-summer colour. In shades of blue, pink and white, there is a variety to suit any scheme, however, it is the deep blue cultivars that are the real stars of the border. Salvia nemerosa Ostfriesland is one of the finest. Growing to only 40cm with dark green leaves and upright spikes of deep bright blue flowers, it needs no support and stays looking good through summer and into autumn. Each tiny deep-blue flower is surrounded by a purple calyx, which remains on the spike and retains colour even after the flowers have faded; thereby prolonging the display.

Salvia nemerosa Carradonna' is a taller variety with deep indigo-blue flowers on dark black-purple stems. Salvias make wonderful planting partners for orange and yellow hemerocallis, contrasting both with the colour of the blooms and the mound of strap-like leaves typical of the day lilies.

The campanulas offer many excellent blue border flowers, from the early dwarf varieties that grace the rockery to the late-flowering statuesque type that bring blue to the back of the border. They also offer a great variety of flower forms. None is more striking than Campanula Sarastro', a sophisticated beauty which flowers in early summer. This is the plant for the front of the border or a raised bed, where it will make a low mat of foliage.

The flower stems rise to 40cm or so and carry large pendant, elongated bells of deep rich blue. It is glorious with low-growing silver foliage plants such as Convolvulus cneorum or Santolina chamaecyparissus.

Campanula lactiflora is a much-overlooked perennial. It produces tall stems up to 1.2 metres or more with loose heads of outward-facing, bell-shaped flowers. There is a number of named varieties but any with soft-blue flowers is pleasing and particularly useful to bring summer colour to shadier parts of the garden. Use it to create a naturalistic effect in the dappled shade under trees in wilder areas.

Everyone likes cornflowers but the annual cornflower can be tall and untidy. Catananche caerulea or Cupid's Dart is a pretty Mediterranean alternative with semi-double, dark-eyed flowers on fine stems above a low mat of leaves. It flowers from midsummer and is useful to add blue to plantings on dry banks and in narrow borders near paving.

Andrew McIndoe