Garden Sense: Flowering shrubs that can take summer’s heat

Opinion June 30, 2012 0 Comments

A girl runs around a growth of Russian Sage at Morris Arboretum (photo by Paul Meyer)

by Anthony Aiello

As summer rolls on and the days seem to get increasingly hotter, many garden plants fade, become scorched or simply stop flowering. During the height of summer in late July and early August, there are very few woody plants whose flowers can withstand the heat and that add much interest to the garden. But there are a few summer-flowering shrubs that love the hot dry weather and add interest to one’s garden.

Russian sage (Perovskia atriplicifolia) is a beautiful addition to any landscape. This is a semi-woody shrub with an upright habit and silvery-gray stems and leaves. Russian sage grows in a dense clump, reaching 3-5 feet tall with an equal spread. It needs full sun and dry soil – if grown in the shade or too rich a soil it will become especially leggy and flop over.

It can tolerate a range of soil types but does not tolerate wet feet, especially in winter. To keep the plants looking their best, Russian sage should be cut back almost to the ground as soon as new leaves emerge in spring because flowering occurs on new growth.

Russian sage comes into its glory in mid summer when it produces spikes of lavender blue flowers, which grow up to 12 inches long and can last for several weeks. The combination of these fine purple flowers with the silvery-gray leaves adds great contrast in a mixed border or as a mass planting. Plants of Perovskia can get large so it is best to use it where it can reach its full size.

There are several varieties of Russian sage available: Blue Mist has light blue flowers and blooms earlier in the season; Blue Spire has darker violet flowers; Little Spire is more upright and less spreading than the species.

Caryopteris x clandonensis, or bluebeard, is another excellent summer flowering shrub, coming into bloom even later than Perovskia. Its bright blue flowers are a welcome addition to the garden in late summer and attract a host of beneficial insects. Bluebeard is slightly smaller in stature than Russian sage, generally reaching 4 feet tall with an equal spread.

Although it can tolerate a range of soils types, Bluebeard requires full sun and very good drainage. Too much moisture is a sure way to kill these plants, which will show some tip dieback in our area, so it is best to prune them hard once there is some sign of new leaves emerging, usually in late March.

I especially enjoy waiting for this plant to come into flower and love the intense blue flowers. Because of its relatively small size, it is useful in a variety of situations, especially when combined into a mixed border.
There are a number of cultivated varieties on the market:

• First Choice with excellent dark blue flowers, compact habit and excellent dark green foliage,

• Grand Bleu™ (‘Inoveris’) – also with excellent dark blue flowers, dense habit, glossy green foliage and flowers in September,

• Longwood Blue, a standard of the industry, larger and more vigorous than other forms, Pink Chablis™ (‘Durio’) – if for some reason you don’t like the wonderful blue flowers typical of Caryopteris, then try this variety with its pink flowers, and

• Sunshine Blue™ (‘Jason’), a form with chartreuse-yellow flowers and light blue flowers – this is an improvement of the older variety Worcester Gold.

If you are looking for a larger summer-flowering shrub, then consider chastetree, (Vitex agnus-castus). If left unpruned, chastetree will grow to 12-15 feet tall, but if pruned in late winter, it can be maintained closer to eight feet. Vitex has palmately compound leaves, providing plants with a medium texture in the garden. It has similar cultural requirements as the previous two plants, that is, full sun and dry soils.

The large spikes of flowers bloom throughout the summer, providing interest for several weeks. There is a range of flower colors of chastetree, although plants with blue flowers are the best performing and most satisfying in the garden. The varieties of chastetree include: Abbeville Blue, Mississippi Blues, and Shoal Creek – all with deep blue flowers – Blushing Spires, a light pink version of this shrub, and Silver Spires, a white-flowered form.

So, if you would like to add some vitality to your late summer garden, think of using one of these plants in that hot sunny location where not much else will thrive. (As always, I need a better closing.)

Anthony Aiello is the Gayle E. Maloney Director of Horticulture and Curator at the Morris Arboretum.

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