Flower gardens with alkaline soil have a pH value above 7.0. The increase in pH causes a reduction in the nutrients available for plant growth, resulting in some species not surviving. Iron is one of the major nutrients that become unavailable due to the high pH level. One benefit of having alkaline soil is that it tends to warm quickly so you can plant your specimens earlier in the spring season. Soils with a heavy Limestone content, or chalky appearance, generally have an alkaline pH. A complete soil test will help you determine the pH value of your garden soil. Contact your County University Extension office for information on having a complete soil test for your flower garden. Make sure you have a separate test for each garden area since the pH may vary based on the amount of fertilizer added to the soil in the past or the types of plants growing in the soil
Amend your garden bed by working in 2 to 3 inches of organic compost 6 to 8 inches into the soil before planting. This will increase the nutrient value and increase the soils ability to hold moisture during hot summer temperatures. Adding organic compost to your garden will change the soil consistency, but does not adjust the soil pH. It is possible to adjust the soil pH with chemical amendments, however this is a time consuming process each spring to keep the pH under 7.0. If you have alkaline soil, and do not want to maintenance of adding chemical amendments each year, check the plant label to verify it will grow well in an alkaline soil with a pH over 7.0.
Trees that tolerate an alkaline soil include; maple, honey locust, Rose of Sharon, eastern red cedar and English oak. All maple tree varieties, including the ornamental Japanese maple, are tolerant. Most maple species and the honey locust grows best in USDA Hardiness Zones 3 through 9. The Japanese maple varieties require the warmer temperatures found in Zones 6 through 9. Rose of Sharon, also known as Hibiscus syriacus, and the English oak prefer to grow in USDA Hardiness Zones 5 through 9. Easter red cedar is a cold tolerant tree that will survive in USDA Hardiness Zones 2 through 9.
Evergreen shrubs that grow well in alkaline soil with a pH over 7.0 include California lilacs, Green Velvet boxwood and Photinia specie shrubs. The California lilac grows best in the warm USDA Hardiness zones of 8 through 10. Green Velvet boxwood are slightly more cold tolerant with a preferred USDA Hardiness Zone of 6 through 8. The Photinia species of shrubs grow best in Zones 7 through 9.
Deciduous shrubs that prefer alkaline soil include Lilacs species, Weigela, Forsythia and Spiraea shrubs. Weigela shrubs grow best in USDA Hardiness Zones 5 through 9, while the Forsythia species grows best in Zones 6 through 9. Spiraea shrubs are a popular variety in the northern areas of the United States since they tolerate colder temperatures in USDA Hardiness Zones 3 through 8.
Perennial plants that prefer alkaline soil include Dianthus species, Clematis, Scabiosa and Potentilla plants. The Dianthus species, also known as Pinks, grow well in the Midwest in areas where the USDA Hardiness Zones are 4 through 10. Clematis is another cold weather favorite since it grows best in USDA Hardiness Zones 3 through 8. Scabiosa and Potentilla species prefer to grow best when planted in Zones 5 through 8.
It is always best to verify the USDA Hardiness Zone of the tree, shrub or perennial you choose to plant to make sure the plant is hardy for your geographical location.
http://www.ipm.iastate.edu/ipm/hortnews/1994/4-6-1994/ph.html
Iowa State University Extension: How to Change Your Soil’s pH
http://web1.msue.msu.edu/monroe/soilweb2/alkaline-tolerant.htm
Michigan State University Extension: Alkaline Tolerant Plants
