If a pH test reveals your soil is alkaline (pH higher than 7), some plants will not thrive there. On alkaline soils certain nutrients, particularly iron, are locked up. Some plants have evolved to grow despite limited access to these nutrients. Other plants—including rhododendrons, camellias and heathers—suffer.
You may try to change soil pH with certain additives, but the results don't last. The better option is to choose plants that like the acidity—or lack thereof—in your soil.
Those who garden on limestone or chalky soil need not despair. More plants thrive in alkaline conditions than on very acidic soil. Other pluses? Most alkaline soils drain well and tend to warm up more quickly in the spring. As with nearly all soils, it's a great idea to add organic matter, such as compost and well-rotted manure, to alkaline soils. This will increase its ability to hold water and nutrients.
Here are some of the great garden plants that prefer alkaline soil:
shown: Green Velvet boxwood (Buxus 'Green Velvet'; Zones 6–8)
Daphniphyllum himalaense ssp. macropodum (Zones 7–8)
Photinia species (Zones 7–9)
Aucuba species (Zones 7–10)
Euonymus fortunei 'Emerald Gaity', 'Silver Queen', 'Emerald n Gold' (Zones 5–8)
California lilacs (Ceanothus spp.; generally Zones 8–10)
Daphne species (Zones 5–8)
Deutzia spp. (generally Zones 6–8)
Forsythia spp. (generally Zones 6–9)
Mock oranges (Philadelphus spp.; generally zones 5–9)
Lilacs (Syringa spp.; generally Zones 5–9)
Weigela spp. (generally Zones 5–9)
Spiraea spp. (Zones 3–8)
Hellebores (Zones 4–8)
Pinks (Dianthus spp.; generally Zones 4–10)
Brunnera macrophylla 'Jack Frost' and 'Looking Glass' (Zones 4–8)
Clematis spp. (generally Zones 3–8)
Potentilla spp. (generally Zones 5–8)
Scabiosa spp. (generally Zones 5–9)
Horticulture's Smart Gardening Techniques also has tips for soil maintenance