Roses have a bad reputation as fussy, fragile plants, and while some hybrids can be maddingly susceptible to insect and fungal damage, most types of roses are not at all fussy. Some, like shrub roses and rugosas, are actually pretty tough cookies. However, like any plant, roses have their preferences and the better able you are to provide them, the better your roses will do. Proper soil preparation prior to planting roses will go a long way toward improving their performance. It can take some time and effort but getting your roses started off in well-prepared soil will help them get established more quickly, grow healthy, and have fewer problems down the line.
The pH is a measurement of the relative acidity or alkalinity of the soil. The pH will affect how well your roses can access nutrients in the soil, so it's worth paying attention to it. If the pH is way off, it won't matter how much you pamper your roses, they will still be stressed. Luckily, roses prefer a soil pH close to the typical level for ordinary garden soil, which is slightly acidic to neutral (6.0 to 7.0). If your soil lies outside that range, as indicated by a soil test, it is easiest to amend the soil before planting, but you can make adjustments afterward. If your soil is extremely alkaline or acidic, you might want to consider replacing it or growing your roses in containers, because adjusting soil pH is not a one-time fix—it requires periodic testing and adjustment.
Roses need a soil that drains well but holds onto moisture long enough for the roots to absorb it. That means that a loam soil is ideal—too much clay and the roots can become waterlogged, but a sandy soil will drain before the roots can get a good drink.
If you are not starting out with loose, loamy soil, you will need to do some amending. To begin, remove any large rocks and stones from the planting site. If you have dense clay soil, don't be tempted to add sand to loosen it up—this is a common mistake that creates a cement-like substance. The key ingredient in making bad soil more friable is organic matter in the form of compost, composted manure, or leaf mold. Organic matter will aid in both water retention and drainage and it loosens the soil texture as it decomposes. It is an excellent amendment for soils that have too much clay as well as those that have too much sand.
Enriching the Soil
Many gardeners like to add additional amendments to the soil at planting time. It's impossible to give specific guidelines on how to enrich the soil for roses—or any plant—because soils vary so greatly. You could have your soil tested or you could take your cue from plants growing nearby. If the other plants in the area are lush, green, and free-flowering, your soil is probably in good shape. However, if they are continually stressed, yellowing, or afflicted with problems, you probably need to add some nutrients to the soil. Rather than getting caught in a cycle of fertilizer dependency, feed the soil with mineral amendments and let the soil feed the plants. This, combined with organic compost or other organic materials mixed into the soil, largely eliminates the need for constant fertilizing. It's not just healthier for the plants, it is less work for you.
While it's impossible to make a specific recommendation for your particular soil, it's usually a good idea to add some phosphorous to poor soils, which helps plants develop strong roots and quickly become well established. Pure phosphorus, or organic phosphorus in the form of bonemeal, is widely available. Mix about 1/2 to 1 cup per bush throughout the soil before planting. Some gardeners like to add kelp or soy meal for added nitrogen, but if you have added organic matter you probably have enough nitrogen for now.
Epsom salts are also in many rosarians' bag of tricks. These add sulfur and magnesium, two elements crucial to healthy plant growth. You can either mix 1/2 cup into the soil or dissolve the 1/2 cup of salts in water and sprinkle around the rose bush. Use caution around the leaves. Any type of salt can burn leaves if used on hot, sunny days.
Once you have the perfect soil for your rose bush and it is settled into the planting hole, add a 2- to 3-inch layer of organic mulch to the surface of the soil around the plant. Mulch will suppress weeds, keep the roots cool, and help the soil retain moisture. Opt for an organic mulch that will slowly break down and continue to feed the soil and improve the soil's texture. Good choices include shredded bark, leaf mold, and good compost.
Spread the mulch all around the root zone of the rose bush, but keep it 2 to 3 inches away from the stem. Piling mulch against the stem can lead to rotting and can provide cover for gnawing rodents and insect pests.
Growing Roses Successfully. University of Vermont Extension