Answer: You may very well be overwatering. Here are a few tips for growing healthy roses in containers:
There are some great types of containers to choose from. Wooden planters, half barrels, decorative plastic pots or planters, synthetic terracotta pots, glazed ceramic pots (with drainage holes), windowboxes for miniatures or small hybrid teas. All are appropriate.
All containers used for planting roses MUST have drainage holes. Most containers sold at garden centers and nurseries do have drainage holes. If not, you can make holes in most plastic pots using a small drill. If you choose terracotta or concrete pots without drainage holes, using a drill that has a special masonry bit works well.
Containers should be at least 14-16 inches in diameter and 18-20 inches deep for most floribundas or small hybrid tea roses. If your containers are larger, that's a plus, as roses really do better in larger containers. Refrain from using containers that are too small as your roses will become rootbound, resulting in poor performance, or worse yet -- death. It's better to have one good-sized container to hold one beautiful specimen rose that will bloom for years, than to have a half dozen smaller containers that will only serve your roses well for one season.
A word of caution about placing plastic type pots with drain holes inside decorative containers without drain holes. You WILL need to regularly empty water that drains into your decorative containers. Roses face almost certain death from root rot if their roots are left in standing water too long.
When placing containers on any surface other than soil, position some "feet" such as casters, blocks of wood, or maybe some small bricks under them. This will help prevent the container from sitting in stagnant water, which your roses won't appreciate.
For optimum growth, using the proper soil is important. There are many soilless mixes that are ideal for growing roses in containers. You can find them at most nurseries or garden centers. These mixes are free of diseases or noxious pests that could harm your plants, and designed to retain more water than other types of commercial mixes.
Before placing soil in your containers, place several broken pieces of clay pots over the drainage holes. Then begin to add soil until your rose will sit in the pot at the correct height. Add additional soil to within 1-2 inches from the top of your container. This will give you the space you need for watering the rose. Press the soil down well to eliminate any air pockets. Finally, soak the soil thoroughly.
You may wish to cover the topsoil with a good quality mulch after planting is completed. If your containers are quite large, consider dressing them up a bit. Just plant a few favorite colorful annual seedlings around the rose. Sphagnum moss or baby's-tears are also great as topsoil covers.
Your containers should be placed where they will get a minimum of six hours of direct morning or midday sun. If you see your roses beginning to lean too far toward the sun, you may need to rotate your containers every few days. To prevent foliage burn from reflected heat, don't place containers near light-colored walls.
Roses get very thirsty on hot days -- just like people! Water your roses at least once a day during the summer. Check the soil in your containers every day to make sure they are getting sufficient water. If the soil feels dry just below the surface, you may need to water twice a day, especially if your outdoor temperature reaches 100 degrees or more. Always check to see that your containers are draining properly.
Roses also need nutrient supplements -- just like people! Container-grown roses need to be fertilized more often than roses grown in gardens. Use a slow-release rose fertilizer and always follow the label instructions.
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