Standards are those shrubs or trees that can be "trained" to show only a central stem. They are forced into a straight plant without lower branches. Purple flowering plants that can be grown as standards can be colorful trees or shrubs. These standard plants can make for a wonderful specimen focal point in the garden. Keep in mind that a soil's pH can make blooms change colors, i.e., hydrangeas are blue/purple in acidic soil and pink in alkaline soil.
French hydrangea, Hydrangea macrophylla, is a deciduous shrub that grows 3 to 8 feet tall. Flowers will be different colors depending on the pH of the soil; for purple blooms make sure the soil is highly acidic. Grow in rich, moist soils in partial shade to sun. Propagate via cuttings.
Crape myrtle, Lagerstroemia indica, is from the loosestrife family. It is an easy-to-grow and fast-growing shrub. It will vary between 18 inches tall and 40 feet tall. Size of leaves depend on the cultivar as do colors (there is a purple variety). Grow in moist soil in good sun. Propagate via hardwood cuttings in winter, seed or medium wood cuttings in fall.
Butterfly bush, Buddleja davidii, is a fast-growing fragrant shrub that is drought tolerant and attracts butterflies. It will grow to 6 to 12 feet high and 4 to 15 feet wide. Leaves are grayish green and 6 to 10 inches long. Flowers are in clusters and in various colors. Purple varieties are African Queen and Petite Indigo. Grow in well-drained soil in full sun to partial sun. Propagate via cuttings.
Purple sage, Leucophyllum frutescens, is from the figwort family. It is an evergreen shrub that is drought tolerant. It will grow 3 to 5 feet high in hot conditions; 5 to 8 feet tall is the norm. Leaves are under an inch; tubular flowers are under an inch wide. There is a purple variety called Rain Cloud. Grow in poor soils in full sun. Propagate via seed and cuttings.