Stake any species that are apt to topple over during the summer
Some widely spreading plants and others that produce large flowers tend to topple over once fully grown. There are several ways of staking plants. Supports made from a metal or string grid or twigs are very useful for plants with several stems starting at the ground. As the stems grow up through them, the new leaves will hide the supports. It is best to use one stake per stem for plants with top-heavy blooms.
Choose plants that are well suited to your growing conditions, because plants that grow naturally in dry, nitrogen-deficient sites will produce long, weak stems when grown in moist, nutrient-rich soil. In addition, many new dwarf and compact cultivars do not require staking. Ask at your garden centre.
Make a grid from string and stakes
This type of support is installed in late May, before plants are fully grown. Place stakes around the outside and in the centre of each plant. Then weave string back and forth between the stakes to form a grid that the stems can grow through. You can also use a metal grid instead of string.
Some plants that can be supported with a grid:
Anthemis tinctoria, Helenium, Paeonia, Solidago, etc.
Stake heavy blooms
Place stakes in the ground early in the season, before the roots have a chance to fill out. Tie the stem to the stake as it grows, adding more ties (in a figure 8 between the stem and the stake) as necessary. Inspect the ties regularly over the summer to make sure that new shoots are not damaged by the string. You can also buy metal stakes with a loop at the top for holding the stem.
Some plants that can be supported with wooden, metal or bamboo stakes:
Althaea, Delphinium, Dahlias, Lilium, etc.
Cut back some species
Some perennials, especially spring-flowering ones, will rebloom if cut back to a few centimetres above the ground after they have finished flowering. This is also a good way of renewing columbine foliage. Avoid this type of pruning in August and September, however, so as not to encourage plants to exhaust themselves before entering winter dormancy. Woody-stemmed plants like lavender and shrub peonies and plants with evergreen foliage should not be cut back.
Some plants that benefit from cutting back:
Anthemis tinctoria, Centaurea, Delphinium, Echinops, Geranium, Lysimachia, Nepeta, Rudbeckia, Salvia.
Thin some species
It is sometimes necessary to remove some stems when a plant becomes too bushy and therefore susceptible to disease. It is usually recommended that you cut one-third of the stems back to the ground once they have reached one-quarter of their full size in spring.
Some plants that benefit from being thinned:
Aster, Delphinium, Lupinus, Monarda didyma, Phlox, Solidago.
Pinch back some species
This type of pruning is done before plants bloom, to encourage lush, bushy growth. A number of perennials can be pinched back. It is usually recommended that you remove the tips of new shoots every three to four weeks until mid-June.
Some plants that can be pinched back:
Aster, Artemisia, Eupatorium maculatum, Leucanthemum, Perovskia atricifolia, Physostegia virginiana, Sedum 'Autumn Joy'.