About sixty species of herbaceous plants, annual and perennial, originating from North America belong to the genus Ageratum; Ageratum houstonianum is a perennial species, native to Mexico, generally cultivated as an annual. It produces small compact, densely branched tufts; the stems are thin, fleshy, green in color, and bear oval, finely serrated, bright green leaves with a velvety appearance; from April to autumn, at the apex of the stems, small blue flowers bloom, similar to small disordered pompoms, gathered in small clusters, which stand out above the foliage. The flowers are followed by small fruits containing seeds. Ageratum houstonianum plants can grow to a height of about 35-45 cm, but there are numerous dwarf cultivars, which remain below 30 cm, which are used in the beds of annuals and in the borders. Among the many cultivars there are also varieties with white, lilac or pink flowers. To encourage abundant flowering it is advisable to remove the withered flowers.
The agate celestino settles in a very sunny place, or in any case where it can receive at least 4-5 hours of direct sunlight every day, to favor flowering; placing the plants in a completely shaded place they will get stunted bloom. Although they are perennial plants, the Ageratum houstonianum, they fear the cold very much and also tend to elongate excessively with the passing of the years; for this reason in Italy they are grown as annuals.
Ageratum houstonianum plants water abundantly and regularly, avoiding letting the soil dry for prolonged periods of time; throughout the vegetative period it is advisable to provide fertilizer for flowering plants, every 15-20 days, mixed with the water used for watering.
The agate celestino prefers soft and humus-rich soils; a good balanced soil is used, mixed with a little manure, and a good quantity of sand, to favor drainage, and to prevent the soil from becoming too compact, not allowing the thin root system of the agate to develop at its best. The agate celestino is also cultivated in pots; let's remember that plants grown in container need more regular care, very regular watering and fertilizing.
The multiplication of this genus to obtain new specimens takes place by seed; these plants are sown in February-March, in seedbeds, to be kept in a protected place until the external temperatures are not mild enough, or in April-May directly at home; to favor germination it is advisable to keep the soil of the seedbed slightly damp.
Celestial agerate: Pests and diseases
Specimens belonging to this tree species are often attacked by aphids or white flies; the cultivation in poorly ventilated places, or the excessively dense positioning of the plants, is often the cause of the proliferation of botrytis, which quickly spoils the foliage.
Before flowering it is good to intervene with the use of specific pesticide products that will allow to provide a correct protection from the attack of these parasites.