Roses are subject to a wide array of pests, diseases and cultural problems, yet they remain one of the most beloved garden plants. While we aim to do our best to care for these garden gems, some cultural difficulties and questions are perennial. We address the most common troubles we encounter in this guide, as well as common diseases and pests on the following pages.
Roses benefit from highly nutritious soil that offers the plant just what it needs for each stage of annual growth rather than relying solely on fertilizer supplements. Feeding correctly takes preparation and then restraint. Begin the growing season with a soil test to make sure that the soil pH is in a range that allows the rose plant to take up the nutrients it needs from the soil. Roses prefer to grow in soil with a pH between 6 and 6.5. If the pH level is off target, any fertilizer you add will be less effective because the pH locks nutrients in the soil. Send a soil sample to your cooperative extension diagnostic lab to get an accurate reading and good advice on what additions should be incorporated into the soil.
Then amend the soil around the base of the roses with some organic compost or well rotted manure to condition it, adding both texture and a complex system of soil components that support healthy rose growth. Fertilize roses each month from April through September for increased quality and quantity of blooms. If the plant is fed too much, you will encourage lots of leafy growth at the cost of flowers. Profuse, young growth is also more enticing and vulnerable to garden pests so you may be inviting problems into your garden by over-fertilizing. Feeding too late in the season will encourage tender growth that can be killed or damaged by early frost.
Roses utilize all fertilizer components continuously through the season but rely more on particular elements at different phase of the growing season: more nitrogen in early spring, phosphorus in the period leading up to bloom and potassium for strength and resilience later in the season. A well balanced food or a good quality, organic, rose-specific fertilizer works well.
Many fungal diseases proliferate in warm wet conditions. Fungal spores present on the plant germinate when wet and then disease is transmitted from leaf to leaf by splashing water. If you can reduce the amount of time that leaves remain wet, you will reduce the germination and transmission of fungal disease. The fungal spores that produce black spot disease, for instance, need to remain wet for six hours to germinate.
Begin by watering by drip irrigation or hand water at the soil level without splashing on the leaves. Water in the morning, when the plant will have the most hours of strong sun ahead of it to dry off any errant splashes (and also to make use of the water through photosynthesis). Keeping plants uncongested and exposed to air circulation will help speed the drying of leaves after rain. Plant roses in areas that receive strong sunlight. Keep in mind that high humidity will slow evaporation making these measures even more important in seasons and areas of very high humidity.
Like many plants, roses talk to us with their leaves. It is natural for leaves to gradually yellow and drop as they age, but more widespread yellowing during the growing season is communicating distress. If you are seeing spots on leaves as well as discoloration, there may be disease at work. Otherwise, It is probably a condition of care that needs to be adjusted rather than a pest or disease. If the leaves on your rose are turning yellow, review their growing situation and consider whether one of these problems could be to blame:
When is the right time to prune roses? It depends on the type of rose you have. As a general rule of thumb, roses that flower only once in a season should be pruned within a month after flowering and roses that flower several times during the season should be pruned in late March or early April once the buds start to break. The precise timing will vary with where you live but you can take your signal from the plants: buds swelling or roses fading. Where and how you make your pruning cuts is also important and proper pruning creates a more vigorous rose with more, healthy flowers. You can read more about when and how to prune different types of roses in our guide to Pruning Roses.
When do many of us have an urge to prune our roses? In the fall when the plants are unruly, with spotty leaves and looking in need of care. But it is better to resist the urge and wait until next year when the effort will have greater benefit for the plant. Pruning too late in the season can, in some cases, reduce the amount of flowers you will have on your roses next years as well as risk damage to the health of the plant. Pruning stimulates a plant to produce new growth; tender growth at the end of the season is at greater risk of winter damage which can lead to further plant die-back during the winter. Spring pruning of repeat-blooming roses also allows the gardener to see which stems are looking strongest at the beginning of the season and select those to remain on the plant.
There are some situations that call for a fall pruning.
The roses at NYBG are mulched with pine bark mulch. Finely-shredded, pine bark mulch creates a nice thick blanket that doesn't get matted down into large clumps. Keep the rose watered up until the time the ground freezes. Mound the mulch around the base of the rose to a depth of about 6 inches in the late fall as the soil freezes. This is what we do to protect the soil for our roses in the Peggy Rockefeller Rose Garden.
In the spring, six weeks before the last frost date, remove most of the winter protection. Leave just a small pile of mulch around the canes. Two weeks later, after the roses have adjusted, clear off the remainder of the mulch and top-dress the roses with compost or (even better) well-aged manure.
Mulch is a stabilizer. It insulates the ground to keep temperature fluctuations minimized which reduces stress on the plant. It also holds moisture in the soil and keeps weed growth down. Shredded organic mulching material breaks down gradually to add texture and nutrients to the soil.
Roses grown in containers have some great conveniences but a few more challenges than those grown in the ground. In winter, the roots are less insulated from the cold as the entire mass of soil will freeze very quickly while the entire plant is exposed to the threat of drying winds. As a result, a container grown rose is at greater risk in a cold climate. You can reduce (but not eliminate) the risk with some protective measures .
Always select a rose that is cold hardy at least two USDA zones lower than your location. If your container-grown rose is located on the terrace of a multi-story building, an even greater buffer of hardiness is necessary to make up for the additional exposure. In New York City where the USDA zone is 7a and 7b, that means selecting a rose hardy to at least zone 5 for a container or zone 4 for a rose in a container on an exposed apartment terrace. If you live in USDA zone 5 or lower, roses are unlikely to survive the winter in a container unless you can temporarily submerge the pot in the soil to protect it. Grafted roses are not a good choice for containers.
Keep the plants watered up until the time the soil freezes and the plants become dormant but do not fertilize to encourage new growth after August.
If you have an unheated garage, shed, barn or enclosed porch, you can protect your container-grown rose by moving the plant into that space after the leaves have dropped and it has become dormant. It is important to choose a space that will shelter the plant from winds and freezing temperatures but will not warm up and break the plant's dormancy prematurely. Sunlight is not needed. Water the rose lightly twice a month.
If you cannot provide a protected space, you can create greater protection for your roses outside. Make sure that the pots themselves are not ceramic or terracotta which are likely to fracture in the cold. Mulch the soil up to the top edge of the pot. Once the roses are dormant, pull the pots into an area near to the warmth of the home and with any wind barriers that are available from the building structure. Create further wind barriers by erecting burlap or tarp screens around the containerized plants. During the coldest weather, you can provide the same protection recommended for roses growing outside of their hardiness zone: surround the rose with a cylinder of chicken wire and sturdy poles. Fill the cylinder to the top with dry leaves or straw and cover it with a solid top to prevent the protective mulch from compressing in the rain and snow. You should also shield the plants from southwest sun which is intense and risks awakening the plant prematurely.
Do not bring the rose into the home or keep it in a place where temperature will rise above the mid-40's during the winter. Keep the plant between 25 and 45ºF.