Roses generally do well clay soils. Growing roses in clay soils successfully depends on how hard the clay is and what variety of rose you are growing. Some roses prefer lighter soils, and gardeners need to remember that roses, like most plants, need soil that is well-drained and has enough organic content to retain necessary water and nutrients.
Clay is often recommended for roses because it holds water, and roses need lots of water. On the other hand, clay often retains too much water, which is not good for roses; roots left sitting in water often develop fungal diseases. Roses can grow in almost any soil that contains organic matter, retains nutrients and drains well.
If your soil is solid clay that packs hard in the summer or gets slimy and gooey in the rain, you might want to amend it with sand and silt to make it easier to promote drainage and make it easier to cultivate.
Whether your soil is sandy, loamy or clay, roses do best in soil that has a pH of between 5.5, which is moderately acidic, and 7, which is neutral. Soil in the West or Southwest is often alkaline. Soil in the Northeast or Northwest is usually more acidic. You can reduce alkalinity by adding sphagnum peat or sulfur. You can correct soil that is too acidic by digging in ground limestone. To find out how to test the pH of your soil contact your local agricultural extension service. Larger garden centers often sell soil-testing kits for this purpose.
Clay soils contain a high percentage of tiny, flat plates of mica and feldspar that adhere closely together. This forms a barrier against water and air. To give the roots of your roses more access to air and water, it is best to break up clay into small chunks and mix in large amounts of coarse-textured organic amendments. Since most of the roots grow near the top, this is where you want to work in amendments.
Organic amendments include shredded bark and chips, manure and compost. Peat moss is good because it contains few nutrients, and it breaks down slowly, helping air and water penetrate the clay for years.
When you add organic amendments such as shredded bark, compost or peat moss to your clay soil, the breakdown requires nitrogen. You can add nitrogen through fertilizers or quality compost; work the fertilizer or compost into the surface of the clay.
Mulch provides a layer on top of your clay to insulate if from the summer heat, preventing it from getting overly hard. Mulch also absorbs falling rain, preventing fungi and bacteria from splashing onto the plants. Worms and insects in mulch create tunnels and paths that help oxygen get to the roots of your roses. The breaking down of organic matter releases nutrients into your soil.
The nitrogen requirements for decomposition are larger if you work the mulch into the clay. Once this mulch becomes part of the soil, simply add more mulch to the surface every year and work it in.