Growing roses in containers offers several advantages not available to those who plant their roses in the ground. At the same time, the techniques for growing roses in containers are not necessarily the same as those for growing earth-planted roses.
Why Containers?
Containers offer several advantages to the rose grower.
(1) If you have no lawn or limited space for rose beds, containers offer a way to have roses, even on concrete surfaces.
(2) Containerized roses do well on balconies.
(3) Containers allow you to position your roses in those small niches where the sun is best, even if it is impossible to make a conventional rose bed at that spot. Please keep in mind, however, that all roses need at least six hours of direct sunshine. Just because a rose is in a container does not mean you can successfully grow it inside on your window sill.
(4) Roses in containers are higher off the ground than earth-planted roses, which makes it easier to work on them if you want to minimize bending and stooping.
(5)You can easily take containerized roses with you at any time of the year if you have to move.
(6) You can avoid the issue of tree roots growing into the place where you have your roses and grabbing up all the nutrients.
What kind of container?
Containers come in clay, stone, plastic, fiberglass, foam resin and wood. They can vary from a minimum of twelve inches in diameter all the way up to a whiskey barrel. The amount of soil the pot will hold is critical. This is determined by both the diameter and the height of the pot.
Some people avoid black plastic pots on the theory that hot black plastic will cook the roots of the roses. However, many rosarians have successfully grown roses in the same black plastic pots that the roses come in from the nursery. Still, clay, stone and decorative plastic pots usually look better than black ones. Wood can also look attractive, but wood usually rots after a couple of years of use.
Soil for container roses
First, do not use indoor potting mix for containerized roses. Potting mix is too porous for use outside, and the roots will dry out and your roses will die. Most commercial rose soils are great for containerized roses, as they have a blend of ingredients that keeps roses from becoming too dry or too wet. Nature's Way Rose Soil Blend was formulated by the Houston Rose Society and contains MicroLife to provide needed nutrients for containerized roses for the first year of the plant's development.
Texas AgriLife Extension Service has had good results on containerized plants by adding 25% expanded shale to the rose soil mix. The easiest way to do that is to place three scoops of rose soil mix into a bucket, then one scoop of expanded shale. Mix. Continue alternating these doses until the bucket is full. Use this amended mix as needed in your containers.
Assuming you buy your rose in a pot (as opposed to bare root), the soil in the pot will take up most of the room in your container. Add rose soil/expanded shale mix to the bottom of the container so that when you place the root ball in the container, the top of the rose's root ball is three inches below the rim of the pot, so you have room to top dress with mulch. Fill any gaps around the edges of the container with rose soil/expanded shale mix, tamping it down firmly. Although some gardeners advocate putting pot shards or gravel in the bottom of your container, other gardeners argue that one can ignore this tradition, so long as you are using a good rose soil (as opposed to gumbo clay) and the container has adequate drainage holes.
Water
Given Houston's heat, the biggest problem with containerized roses is providing them with adequate water.
Containers dry out fast. This is especially true if you set your container directly on hot concrete. In June, July and August, it is usually necessary to water twice a day for best results. If you go out of town for a weekend, your roses may die without water. The solution is to make sure your container roses are on a water system. Drip systems are the most efficient way to do this. The aim is a gallon of water each morning (and another gallon in the afternoon during the summer). Because roses do not like wet feet, you must make sure that your container has a hole of at least 1/2 inch diameter in the bottom. Larger containers require additional drainage holes.
Fertilizing
Roses are heavy feeders. That is true even if they are in containers. Rosarians should use caution in employing chemical fertilizers to avoid the salt buildup that can occur in containers. Using organic fertilizers sidesteps this problem. Because of the limited amount of soil, fertilizers should be applied at one-half of their recommended rates. Liquid applications are most practical.
Pruning
Prune containerized roses just as you would an earth-planted rose. In Houston, we prune about Valentine's Day. Remember that you want to keep the size of the bush in proportion to the size of the container.
Root Pruning
There is a definite tendency for containerized roses to become root bound. After all, the roots never stop growing, and the container is a fixed size. An easy solution to this dilemma is to prune the roots at the same time you prune the bush. It is usually pretty easy to lift the plant and intact root ball out of the container. If the root ball is stuck, try running a knife around the inside edge of the pot. That should loosen the soil from the pot and allow you to carefully lift out the rose.
Before removing the root ball, note whether the top of the soil level is lower than three inches from the top rim of the container. Usually, this will be the case because the organic matter in the soil will have decomposed during the past year. The goal is to add fresh rose soil/expanded shale mix to the pot each year to replace the volume that disappears as a result of decomposition and is washed away by the frequent watering.
If you cut off the bottom inch of the root ball (assuming your container is at least ten inches in height), there will be additional room for fresh rose soil/expanded shale mix. Also, if you cut off no more than one palm sized sliver from the sides of the root ball, you can add rose soil/expanded shale mix in the gap that will be there when you replace the trimmed root ball back into the container. Be sure and tamp the soil down with your fingers as you pour it in the gap. Add three inches of mulch on top and you are ready to put the dripper back in place and enjoy another year of roses.
Recommended Roses
Smaller roses, such as miniatures, are suitable for your twelve and fourteen inch containers. Larger roses, such as hybrid teas, will require pots with much larger diameters.
Generally, most miniature and mini-floras will work in twelve or fourteen inch pots. Polyantha and China roses are often small enough to work in pots. So are some floribundas. Even certain hybrid teas are suitable for containers if the pots are big enough. With container roses, bigger really is better. A list of specific rose recommendations by type, color and fragrance, is available on the Houston Rose Society website, www.houstonrose.org under the FAQ button.