Spring comes late to Caithness. Grass grows at temperatures above 8.5^oC and this temperature is not regularly attained until mid-April.  And since there are no big areas of woodland, there are none of those plants that put their flowers out early to get the benefit of the light before the leaves come on the trees.

There are some prominent aliens, which have been planted in the wild to brighten up the spring. Notably daffodils and snowdrops have been introduced. The daffodils prosper but do not spread, while the snowdrops have spread and produce wonderful displays under trees.

Butterburr - Butterburr is an invading alien flowering in February.
Another spreading alien that is perhaps less appreciated is the white butterburr (Petasites albus) now present in substantial colonies in Wick (below the Assembly Rooms), Castletown (West end of the beach), Thurso (Riverside walk) and Latheronwheel (by the old bridge).

Coltsfoot - Coltsfoot is a native plant flowering in March.
Coltsfoot (Tussilago farfara) is a native wild plant which puts out its flowers in March and later its leaves will emerge with the outline of a colt's footprint. It is a widespread coloniser of disturbed ground.

Lesser Celandine - Lesser Celandine has heart-shaped leaves with a dark mark.
The first of the buttercup family to flower is the lesser celandine (Ranunculus ficaria).  It occurs in two sub-species in Caithness - the most common is that which produces little bulb-shaped tubers in its leaf axils and spreads by them being spread around - Ranunculus ficaria ssp. bulbilifera, while the less common and more convincingly natural one is Ranunculus ficaria ssp ficaria which lacks these tubers and spreads by seed in rough grassland habitats.
 

Primrose - Primroses prefer grassy slopes and flower early.
The common primrose (Primula vulgaris) shows its flowers early on grassy banks.

Catkins - Male catkins of the Goat Willow make an early show.
Trees and shrubs often put out their flowers early, before the leaves. Willow, birch, alder and hazel put out catkins, often of a single sex; the male ones are yellow with pollen, while the female ones often less prominent.

Ajuga -  The Pyramidal Bugle is nationally scarce and flowers in May.
And one special plant in the North that flowers in mid May is the pyramidal bugle Ajuga pyramidalis. I know of it in the Thurso river valley and in the Langwell strath, but there are old records for the Reay area, the Shebster/Halkirk area and Berriedale. Can anyone re-locate these? It is a nationally scarce plant with its main population in West Sutherland and the Western Isles.

Mayflower - Found In ditches and damp grassy places in May.
The mayflower or cuckooflower, (Cardamine pratensis) is aptly named and brightens up the month in ditches and damp grassy places