This Plant Crib by the Botanical Society of the British Isles in association with National Museums & Galleries of Wales describes the differences between the Polygalas described here

This describes the difference between Polygala vulgaris and Polygala serpyllifolia

Milkwort Family:-

"Low, often prostrate, slender, hairless, variable perennials, with leafy stems, the leaves unstalked, laceolate, usually alternate. Flowers in short spikes, oblong, about 1/4 inch long, of various shades of blue, pink or white; sepals 5, the 2 inner large and petal-like on either side of the smaller 3-5 true petals; stamens prominent, petal-like, in 2 bundles of 4, joined in a Y. Fruits flat, inversely heart-shaped, slightly winged." from Collins Pocket Guide to Wild Flowers by David McClintock and R.S.R. Fitter assisted by Francis Rose - ISBN 0 00 219363 9 - Eleventh Impression 1978.

Milkwort Family with its Common Name - Botanical Name. Flowering Months Range. Habitat with link to that Loosestrife to Olive Wild Flower Families Gallery:-

Common Name

Botanical Name

Flowering Months

Habitat

Chalk Milkwort

Polygala calcarea

April-June

A perennial herb found in tightly-grazed chalk and limestone grassland, usually on warm S.-facing slopes. It is a poor competitor which disappears if insufficient grazing allows coarser grasses to become dominant. Lowland. Polygala calcarea 'Lillet' has RHS Award of Garden Merit.

Gentian Blue Flower 8 May

White Flower from Detling in Kent in June

Foliage 22 May

Form 22 May

Common Milkwort

Polygala vulgaris

May onwards

A perennial herb which usually grows in short, moderately infertile neutral to basic grassland on banks, hill-slopes crags and sand dunes. It also occurs in acid grasslands, heaths and fen-meadows. 0-730 m (Mourne Mountains, Co. Down).

Flower June

Flowers from Kynance Cove on 25 May

Flowers from Walderslade in Kent in June

Form from Doune Bank on 19 May

Heath Milkwort

Polygala serpyllifolia

May-September

A perennial herb occurring on acidic soils in grasslands, moors, heaths and mires. 0-1035 m (Ben Lawers, Mid Perth). This plant is food for the Small Purple-barred Phytometra viridaria moth.

Flower from Dartmoor in May

Flowers from County Clare in June

Foliage from Dartmoor in May

Form from Sutherland on 3 August

Kentish Milkwort

Polygala austriaca

(This is the same plant as Polygala amara below)

May-June

Grassland (in open chalk grassland, seen only in Kent)

Yorkshire Milkwort

Polygala amara

(Polygala amerella, Polygalla uliginosa)

May-July

A perennial herb of base-rich substrates. On the North Downs it is usually found in close-grazed chalk grassland, and in N. England in limestone grassland, on rock ledges and in fissures in limestone scars, and sometimes on damp stream banks and in areas of eroded sugar limestone. Generally lowland, but reaching 530 m on Cronkley Fell (N.W. Yorks.).