Approximately 200 species of annuals, perennials and subshrubs mainly from North America with about 18 species divided between Central and South America, Asia and Europe. They are partially parasitic (hemiparasites) on the roots of associated plants including grasses. In general they are erect wiry plants, the perennial species often tufted with alternate, simple or lobed leaves and dense terminal spikes of flowers, each bloom in the axil of a conspicuous, often vividly-coloured bract. The flowers which are comparatively inconspicuous and more or less hidden among the bracts, tubular, two-lipped, the upper lip often elongated into a beak-like structure called a galea.
Although this genus is renowned for the very colourful paintbrush-like flower spikes many of the species produce, its parasitic nature has resulted in horticultural neglect. What is most needed is a list of the showier, dwarfer species and the plants that they grow close to in the wild. Travellers in the home territories of the castillejas, please note. There is nevertheless plenty of scope for experimentation in the garden, either using potted specimens of known or presumed host plants, or scattering the tiny seeds around a wide range of rock plants in the hope that some will make a compatible union. The amount of extraction from the host plant seems to be small and casual observations suggest that little harm is done. At least some species seem able to grow to flowering size without a host, though the plants are generally smaller than usual: some at least have grown well in gardens given peat-bed conditions. Most castillejas are native to dry warm areas with freely draining soils but there are some exceptions. Propagation is entirely by seed. (The following selection is largely of North American species seen in the wild and mentioned over the years in the pages of the A.G.S. Bulletin)