Melissa - A good herbaceous and herbal foliage perennial. The Lemon Balm being most popular. easy to grow, and very few pests.
Miscanthus - Miscanthus Ornamental Grass A very desirable group of grasses - both for their summer flowering and also the extended season of interest which is well into the winter.
Molinia - Ornamental Grass A few of the Molinias are good for use with other perennials, and also help to extend the season of interest into the winter with their attractive - if dead - foliage and flower spikes.
Muscari - Grape Hyacinths - colourful blue spikes for the front the border on this low growing bulb, or in open areas of the shrub bed. Gives good early spring colour - blue.
Macleaya - Plume Poppy - The plume poppy gets its name from the long stems of flowers with a plume-like appearance. All of the perennials are rhizomatous; all have attractive foliage ranging from grey green to glaucus. Most soils and situations will suit, but dry and sunny is best.
Maianthemum - The May Lily is low growing, creeping by rhizomes with interesting heart shaped leaves. A member of the Lily of the Valley family, they are happy in dappled or full shade.
Malva - Typical Mallow flowers, but here on hardy perennials. Most have a woody base, from which the young growth sprouts. Flowers can be white, pink or blue dependent upon variety.
Matthiola - Stocks - Not a long lived plant, but a few years of good flower normally available. Be sure to select the hardy perennial types.
Meconopsis - Surprisingly easy to grow, but best treated as biennials. A wide range of poppy flowers with the blue Meconopsis grandis being the most coveted.
Meehania - Flowers from late spring through until early summer. Deciduous with heart shaped leaves and purple cerise flowers held aloft on short stems. dappled shade suits best, and even a woodland setting if it is in moist soil.
Megacodon - This is an interesting perennial if you can grow it - now there's a challenge! Basically it is an upright, tall, Gentian! It grows up to 2m tall, and is reputedly hardy! (Investigation required here, because I do not know it.)
Melica Ornamental Grass - herbaceous (deciduous) perennial grasses i9n an assortment of sizes. Most are clump forming with ranges of height from 30cm to 1.5m. Spikelets of flower on long slender sometimes arching stems.
Melittis - Commonly known as the Bastard Balm. Not sure what it has done, or what its parentage is to deserve that! But for all that, it is an attractive upright flowered perennial preferring a moist soil.
Mentha - Mint - Not only a herb, but also some interesting types for borders and containers - not least being the varegated form of Mentha sauvolens Variegata, which has silver-white markings.
Merendera - Similar in most respects to the Colchicums, and is from the same family. The flowers are less showy, and tend to be star shaped clusters rather than the bowl shape of the Colchicums.
Mertensia - A clump forming group of perennials for a damp area where they can also get dappled sun. Similar in habit to Borage group, and same family. Some upright, some clump forming and some creepers, so choose wisely.
Meum - Baldmoney being the common name of this alkaline loving perennial. Ferny foliage with creamy white flowers. (It is one of the carrot family).
Michauxia - Short-lived perennial, but well worth growing if you have alkaline soil. The white flowers are of the Campanula family, but are recurved upwards on good strong stems.
Milium - Ornamental Grass - A good semi-evergreen golden foliage grass with a height and spread of around 40cm.
Mimulus - The Musk or Monkey Flower - is a wide range of perennials with variable hardiness. Normally low growing with bright, attractive widely tubular flowers with a pronounced 'lip'. Normally require a damp soil and its fair share of sunshine, but also suits as a container plant. Have also seen them flourish in dry rock gardens.
Mirabilis - Tuberous perennial which will need some winter protection, but well worth the bother. (You can simply lift the small tubers in Autumn and re-plant in spring). Not unusual to see several differently coloured flowers on the same plant.
Mitella - The Bishop's Cap - One of the Saxifrage family so well suited to woodland or semi shaded areas. A good ground cover plant in that situation. Individually the flowers are not spectacular, but as a clump or drift they make a food show.
Moltkia - A good borage type perennial for alkaline soils. Similar to and often named as Lithospermum.
Moluccella - The Bells of Ireland - short lived so will need periodic re-seeding, but a good addition to most perennial area - or open spaces in shrub borders. Green flower bracts, and the dead plant persists well into winter for added interest.
Monarda - Bergamot - Herbaceous perennial which also has herbal uses - mainly aromatic oils. Pink shades and deep red are available for this upright growing group.
Morina - Evergreen perennials with upright stems of tubular flowers. Mornina persica - at 1.2m tall - is quiote spectacular with its light pink flowers, shaded deep pink at the base.
Morisia - Hardy, but avoid the winter wet. Dry rock garden or well drained container will suit.
Myosotis - Forget-me-nots - are of course biennial and not perennial. But, once you have them, you will always have them as they re-seed themselves into permanence.
Myrrhis - Sweet Cicely Herb - Ferny aromatic foliage
is the main focus, the white flowers being an added attraction. However
if you want for the herbal uses of the leaves, then you will remove the
flower heads as they appear.