Polygala 'Purple Passion' The beautiful Polygala (Milkwort) ‘Purple Passion’ will really liven up your beds and borders. This hardy plant brings a veritable ocean of purple-pink flowers with bright yellow cups during its flowering season of March through to May - an enchantingly lovely sight to behold. This plant has a wide reach and grows to around 20 cm high. Will also look terrific in pots and planters on your patio. Polygala is hardy to -25 centigrade and smells just delicious.