Soils with a pH level that is higher than 7 are said to be "alkaline." Such soils are suitable for growing plants that thrive in a "sweet" soil, as opposed to a "sour" or acid soil. If soil pH needs to be raised (meaning the ground is not alkaline enough), apply garden lime. If, on the other hand, your soil has too much alkalinity, you can lower the pH by applying a fertilizer that has sulfur/ammonium-N in it (you may see "Ammonium sulfate" on the label). Do not be scared by the chemistry lingo: When you are at the garden center, just look for a fertilizer intended for acid-loving plants (it will have the ingredients that you're seeking).
Fortunately, just as there are plants that like acidic soils, which give you planting options on sour ground (when you can't raise the soil pH or do not wish to bother doing so), so there are plants that like alkaline soil (or, at least, do not mind growing in it). Observe, however, that even plants within the same genus can "disagree" over what kind of ground that they like to grow in. Take the magnificent lady slipper orchid (Cypripedium), for example. There are many types. Some like their ground sweet, others like it sour, and still others prefer a soil pH that is somewhere in between. But, generally speaking, the following types of plants are good choices to grow in alkaline soils.
Plants That Grow Well in Alkaline Soils
The list below, from the Royal Horticulture Society, the UK's leading gardening charity, is not exhaustive, but it gives you enough options to begin planning to landscape on ground that is alkaline. You will find perennials, vines, shrubs, trees, and annuals on the list. A plant's inclusion on this list does not necessarily mean that it needs or prefers to grow in an alkaline soil (although it might), only that it will, at the very least, tolerate alkalinity:
What Are Perennials?
Perennial refers to a plant that is expected to live for more than two years. While some perennials keep their foliage year-round, the top portion of other perennials dies each winter and regrows in the spring via the same root system.
We are breaking this list into sub-lists: perennials (including mentions of a bulb plant and a couple of types of ornamental grasses), vines, shrubs, trees, and annuals. We will begin with perennials, which are the first plants that people think of when the subject of flower gardens comes up. By paying careful attention to sequence of bloom in your planning, you can experience many months of magnificent floral color in your yard by growing perennial flowers:
Bearded iris (Iris x germanica)
Black-eyed Susans (Rudbeckia hirta)
Candytuft (Iberis sempervirens)
Columbine (Aquilegia spp.)
Daylilies (Hemerocallis spp.)
Easter lilies (Lilium longiflorum, a bulb plant)
Festuca glauca 'Elijah Blue' (an ornamental grass)
Foxglove (Digitalis spp.)
Goldenrod (Solidago spp.)
Lavender (Lavandula spp.)
Maiden grass (Miscanthus sinensis, an ornamental grass)
Reticulated iris (Iris reticulata)
Shasta daisy (Leucanthemum x superbum 'Becky')
Woodland phlox (Phlox divaricata)
Yarrow (Achillea millefolium)
Vines, and particularly flowering vines, are remarkably useful plants in a landscape. The one drawback they have, as a class, is that many of them are aggressive. So if you are someone who, in your plant selection, strives to obtain plants that are compatible with low-maintenance landscaping, make it a point to research the qualities of a vine carefully before purchasing it:
Boston ivy (Parthenocissus tricuspidata)
Virginia creeper (Parthenocissus quinquefolia)
Winter jasmine (Jasminum nudiflorum)
Shrubs have been dubbed the "backbone" of a landscape because they furnish it with valuable structure. Select a variety of flowering shrubs if you wish to optimize color in your spring and summer landscape; some are also good shrubs for fall color. But also remember that the value of evergreen shrubs soars in winter when all of your other plants have dropped their leaves. When it comes to alkaline soil vs. acidic soil, Hydrangea presents a special case. You can change a hydrangea's color by changing the soil pH.
Some types of shrubs that will grow in alkaline soils are:
Arborvitae (Thuja spp.)
Contorted filbert (Corylus avellana)
Lilac bushes (Syringa spp.)
Rose of Sharon bushes (Hibiscus syriacus)
"Tree" peonies (Paeonia suffruticosa)
Yew bushes (Taxus spp.)
Because of their size (and because of their corresponding cost and impact on your property), you have to pay special attention to the plant-selection process when choosing a tree. But if you get it right, you can end up with a plant that you will later deem indispensable to your landscape. Some are towering giants that can cast shade over a large portion of your yard, while others are much shorter and serve effectively as shade trees for patios. Still others are considered dwarf trees and function more like shrubs in your landscaping:
Common (or "European") beech (Fagus sylvatica)
European ash (Fraxinus excelsior)
Horse chestnut (Aesculus spp.)
Mugo pine (Pinus mugo)
Ornamental cherry (Prunus spp.)
Some annuals can also be grown in an alkaline soil without difficulty, including Calendula, bachelor buttons (Centaurea cyanus), and sweet alyssum (Lobularia maritima). Mix some annuals into your perennial flower borders to achieve the landscape color scheme that you are seeking.