Individual plants make an impressive single statement in the border, (top left) and the Allium illustrated is 'Globemaster'.
Alliums are made to mix with grasses and look good with Achillea (2nd image centre). Alliums are predominately purple flowering although there is a blue form, careruleum, and a very attractive yellow variety, Allium moly (golden garlic) which has umbel type flowers in the summer. Alliums have an additional bonus that the flower heads look good as they fade and as the seed heads form, illustrated below right.
If you are combining with other plants check the variety to ensure the flowering times coincide. To plant the combination of grasses and Alliums illustrated centre above and below left it is essential to plant a late flowering Allium such as A. sphaerocephalon so that it is at its best when the grasses are looking good and the Achillea in flower.
Alliums are members of the onion family and the small Allium schoenoprasum, Chives, are good to eat and make a lovely edging plant front of the border and are wildlife friendly.
Allium are illustrated here:-
A. cristophii (image top right) is short at 60cms with spiky flower head which will self seed in the right conditions;
A.hollandicum (image below) 'Purple Sensation' tall 80cms dark purple;
A. Globemaster 100cmc with large deep purple flower heads.
Centre image above is Allium sphaerocephalon, a late summer flowering variety. It looks good planted with grasses which are at their best in late summer and early autumn, and also with late flowering perennials such as Rudbeckia and Helenium.
In the third image above is the very attractive A. christophii which flowers in the late spring and early summer, with spiky flower heads which look very good as they fade and dry.