If your yard is covered with pine needles from several mature pine trees, use the needles as mulch rather than bagging and discarding them. According to the Ohio State University Extension publication, "Mulching Landscape Plants," pine needle mulch increases acidity in soil. Planting acid-loving flowers and shrubs that benefit from the pine needle mulch reduces your work and benefit the plants.
Flowers that love acidic soil include Lily of the Valley, dianthus, yarrow, iris, orchid, pinks, rue, hosta and veronica. Some of these flowers, such as hosta and Lily of the Valley, prefer the shade under pine trees. If there is sufficient room between the tree's roots, you can plant these flowers directly to the soil. Mature pine trees have large roots that are difficult to plant around; build a raised bed for flowers under these trees.
Shrubs that benefit from pine needle mulch include azalea, gardenia, hydrangea, jasmine, rhododendron, bayberry, Daphne rose and holly. Gardenias need a monthly application of an acidic fertilizer; replace it with a layer of pine needles smoothed around the base of the shrub. Gardeners use an acidic fertilizer to turn hydrangea's flowers blue; use pine needle mulch under the hydrangeas instead.
Amending the Soil
To determine whether the soil's pH needs to be amended, test the soil in the garden. Take a sample to the local agriculture extension agency or use a soil testing kit purchased from a gardening center.
To reduce the pH by 1.0 points, The Garden Helper recommends mixing 1.2 oz of ground rock sulfur for each square yard of sandy soil, or 3.6 oz per square yard for all other soils. Amend the soil two to four weeks ahead of planting to allow the amendments time to adjust the soil's pH.
Getting Pine Needle Mulch
If there are not any pine trees in your yard, buy pine needle mulch--also known as pine straw--from home garden stores or nurseries.
If you live in an area with pine trees, watch for neighbors who rake and bag their mulch to put out for the trash. Ask if you can have the needles.
Using Pine Needle Mulch
Once the flowers or shrubs are planted, spread the pine needles around the base of the plants. Keep the pine needle mulch between 2 and 2 ½ inches deep. Too shallow a depth will not control the weeds or help the soil retain moisture. Mulch that is too deep keeps the soil too wet, causes disease, rots the roots and attracts rodents and other pests.
According to Purdue University Cooperative Extension Service's "Landscape Plants for Acid Soils", test the garden's soil every three years, as "alkaline water supplies will cause the soil to become less acidic."