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 (that is, 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 -- or do not want to -- raise the soil pH), 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 Will Grow Well in Alkaline Soils
The list below does not pretend to be exhaustive (source: Royal Horticultural Society). But it gives you enough options to begin planning to landscape on ground that is alkaline. You will find perennials, vines, shrubs, and trees 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:
I am breaking this list into sub-lists: perennials (including mentions of a bulb plant and a couple of types of ornamental grasses), vines, shrubs, and trees. 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:
Easter lilies (bulb plant)
Festuca glauca 'Elijah Blue' (ornamental grass)
Maiden grass (Miscanthus sinensis, an ornamental grass)
Vines -- and particularly flowering vines -- are remarkably useful plants in a landscape. The one drawback they have, as a class (if one may generalize about them), 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:
Shrubs have been dubbed the "backbone" of a landscape, because they furnish it will 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. Some types of shrubs that will grow in alkaline soils are:
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 are considered dwarf trees and function more like shrubs in your landscaping:
Where to Go From Here
Some annuals can also be grown in an alkaline soil without difficulty, including Calendula, bachelor buttons, and sweet alyssum. Mix some annuals into your perennial flower borders to achieve the landscape color scheme that you are seeking.
If you are interested in the topic of acidic soil versus alkaline soil, you may also want to read about how to change a hydrangea's color to purple by changing the soil pH.