Umbel

Normally flat-topped inflorescence in which the pedicels or peduncles all arise from a common point

Umbellate

Flowers in umbels

Undercutting

The operation of severing downward growing plant roots in situ, to control root development; usually by machine

Understock

The rooted portion of a plant used for propagation by budding or grafting

Union

In a worked plant, the junction of scion and rootstock

Unisexual

Of one sex