The sensitive foliage and colourful flowers of yearly cosmos crops (Cosmos bipinnatus) offer interest all summer to the house landscape. These tall plants prosper in Sunset’s Environment Zones 1 through 24, making them a suitable addition to those in drier inland areas and to equally moist gardens. Although not required, cosmos stay healthier with reasonable pruning and bloom profusely, seem better. A trim eliminates the less desirable stems and foliage and encourages development.
Pinch back when the crops are between 6 and 8″ tall or the vertical expanding stems after transplanting. Remove the suggestion 1/4 inch above Pinching forces the stem while reducing legginess, resulting in fuller development and flowers.
Cut back the whole cosmos clump all the aged flowers have gone to seed and following the first flush of spring bloom is finished. Remove the development that is best on every plant with sharp shears to inspire flowers or new development.
Remove spent blossoms for the the rest of the summer-flowering period. Pinch the flowers off independently or cut back the flowering stems to within 1/4 inch of a leaf.
Inspect the cosmos through the entire season. Trim out illnesses, broken or dead leaves and stems as required. Cut these back with their base therefore ailments do not spread to plant components and dump the foliage.