Here are answers to some of the most common questions we receive about garden plants. You will find concise information on general gardening techniques as well as plant selection and care. For detailed information on specific plant pests and problems refer to our Common Garden Pests and Problems page.

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Horticulture Questions and Answers

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Which hydrangeas grow well in St. Louis?

Hydrangeas are medium to large shrubs noted for their small, clustered flowers that are often used in fresh or dried arrangements. The key is to let the blooms get fully mature before cutting. For dried arrangements, leave the water level low and let it run out --the blooms will dry in an upright position or clip and bundle with a rubber band to let dry upside down in the garage.

Smooth hydrangea (Hydrangea arborescens) is a native shrub to Missouri. It produces white blooms in early summer and likes some shade where the soil remains moist. It is subject to wilting in dry soils, so avoid west exposures where it's hot. These plants bloom on new growth, so can be cut back each season. 'Grandiflora', otherwise called hill-of-snow, is a popular cultivar. Similarly, ‘ Annabelle' is another good choice with bigger and more numerous blooms. For both, some deadheading will prolong the bloom into fall.

Panicle hydrangea (H. paniculata) produces pyramidal clusters of white flowers in mid-summer that turn slightly pink as they mature. The peegee hydrangea, H paniculata 'Grandiflora', can be trained to a tree form and is quite popular as a specimen tree. The 'Tardiva' cultivar is a good choice blooming from early to late fall. Panicle hydrangeas should be grown in moist soil and away from the hot afternoon sun. Like the smooth hydrangea, flowers are born on new wood so plant height can be controlled in late winter or early spring. Non-flowering dead wood should be cut out each year. Deadheading old flowers will encourage prolonged bloom.

Oakleaf hydrangea (H. quercifolia) also produces long pyramidal-shaped, white flower clusters in mid-summer. These age to a purple-pink form and contrast with the rather coarse, but handsome oak leaf-shaped, dark green foliage. Fall leaf color is brilliant red and orange. The bark is also attractive with a cinnamon color exposed under peeling outer layers. These plants tolerate full sun, but appreciate cool spots for the afternoon. The species will grow to 8 feet tall as well as other cultivars including, 'Snowflake', 'Snow Queen' and' Alice'. A new dwarf form called 'PeeWee' grows only to 4 feet, appropriate for smaller landscapes. Pruning should be done just after bloom since flowers are produced on 2-year-old growth.

Florist's hydrangea (H. macrophylla) is a common pot plant sold at the grocery store with large blue or pink blooms. You can attempt to grow them outside after indoor use, but their hardiness is always an issue. They need to be protected from the winter. There are two types of big leaf hydrangeas --mophead types have rounded flower clusters and lacecap types are flat. A couple good mopheads are 'Nikko Blue' and 'Forever Pink'. Likewise, some recommended lacecaps are 'Blue Billow' and 'Blue Wave'. The root systems are perfectly hardy here, but the stems are commonly killed back due to cold winter temperatures. The lack of flowering draws notice amongst gardeners and can be attributed to either the flower buds being wiped out by the cold or by improper pruning. Since they flower on old wood, cutting them back in the fall "chops" the potential for flower production the next year. The right thing to do is cut out some of the old stems to thin, then control height just after flowering in mid-summer. If you wait till fall, the flower buds have already been set and you will jeopardize the flower crop the next year. Flower color is also an issue for gardeners. This is controlled in part by the soil pH. Acidic soils with pH values of 6.0 and lower will give blue blooms while pHs above 7.0 typically produce pink blooms. You need to have your soil tested to know how much sulfur is necessary to lower the pH or how much lime to apply to raise it.