Tradescantia pallida 'Purpurea'
Sprawling ground cover.
Tradescantia sillamontana
Very drought tolerant, in fact takes all that the summer months can throw at it but suffers badly in the winter, more from the wet than the cold I suspect.
Tropaelum majus
Allowed to self-seed at will, especially in our 'orchard' area as they provide colour in the spring and can be cleared away as they die down in the summer.
Tulbaghia violaceae
No U, so on to V:-
Verbena bonariensis
Another self-seeder that is allowed to get away with it in certain areas.
Viburnum suspensum
Grown in partial shade and given regular water.
Viburnum tinus
One of the unsung heroes of Mediterranean gardens, an ever reliable standby that flowers each winter with little attention, or water.