Chaste tree (Vitex agnus-castus) is a deciduous shrub that forms spikes of lilac blue or pink flowers in late May and will continue to flower for several months. It is a great shrub to plant if your goal is to bring more pollinators to your yard. A fast-spreading plant, chaste tree can reach maturity in three years, producing 2 feet of growth annually, Adrienne Roethling says.