How to grow flowers and foliage plants in containers
The following is my advice on growing ornamentals in containers for outdoor use (Growing vegetables in containers can be found here).. Be advised that techniques can differ based on your geographical location, the type of plants being grown and whether they are indoor or outdoor containers. I grow in zone 5, in the central new york area.
Choosing your Container
Choose larger containers whenever possible so you don't have to water them as often. I try and stay away from unsealed clay pots as they absorb a lot of water. I still love the look of clay though, so you can soak them overnight in water before use and that will help retain moisture. You can also seal the outside of new clay pots with a specially formulated clay pot sealer or by brushing on clear polyurethane. Painting the pots with acrylic paints will also help to seal the pots, though you will want to seal the paint also to preserve your paint job.
Containers come in a variety of materials, shapes and sizes such as clay, plastic, concrete, ceramic, wood and resin. I prefer plastic and resin as they are lightweight, relatively inexpensive, last for many years and come in hundreds of shapes, sizes, colors and designs. Whatever you choose, make sure there are adequate drainage holes. Many resin containers do not have holes drilled in them so you can use them for other things, such as water gardens. They are very easy to drill, however, with a standard ¼" - ½" drill bit. They also sell containers that are self watering which can come in handy on hot days or while on vacation.
When using especially large pots, take into consideration what they will weigh when full and if they will need to be moved once you set them in place. If there is any chance you will have to move them, I'd suggest putting them on a plant dolly before filling the container with soil and plants. Plant dollies are usually flat and made of weather resistant wood or metal and have four caster wheels on them. They come in different sizes depending on your pot and are usually not visible when under the container. Make sure the weight rating on the dolly can handle the weight of the container when full of soil, plants and water.
Remember that most anything that can hold soil can be used as a container for plants, providing it has proper drainage holes, so use your imagination and have fun with it.
Choosing Your Growing Medium
Soil for containers should be lightweight and airy to allow the roots of your plants to get the oxygen and moisture they need to be healthy. Soil from your garden is too heavy and should not be used.
I tend to buy prepackaged potting soil in large bags at my local garden center or discount store because it's easier for me, though I know many people who like to mix their own potting soil. I also like to add 1 part compost to 4 parts potting soil to my containers as I feel it helps to retain moisture better than potting soil alone without smothering the roots. The compost also adds important nutrients and organisms to the soil that plants need to survive.
Keep in mind that some plants may require special soils such as cacti, vegetables, and orchids so the plant's needs should be researched before planting, but generally a potting mix containing equal parts peat moss, compost and builder's sand will work for most plants. When making your own soil mixes, it's important to add a slow release fertilizer to the mix, following the manufacturer's instructions on the fertilizer label. I like to use organic slow release fertilizers as they are less likely to burn your plants and they will last for a longer period of time. I have a whole page dedicated to making your own soil mixes and fertilizers that you can check out here.
How to Plant
When planting your containers, choose plants that need similar light, water, and fertilizer requirements. You wouldn't want to plant cacti with petunias. Though they both like sun, the cacti like it dry with little fertilizer and the petunias need lots of water and fertilizer. Most annuals work well in containers, but don't rule out perennials. Just know that if you live in a colder climate, you may have to transplant perennials into the garden come fall because they most likely will not survive in the container all winter.
Choose plants with varying heights and textures and complimentary colors for your containers. And don't forget about foliage plants. They will add interest when the flowering plants are between bloom cycles.
I like to plant in groups of odd numbers. For example: A tall “focal point" plant will be in the middle surrounded by three evenly spaced medium sized plants, with three low growing or trailing plants between those. I also like to design my containers the way I would design a vase full of flowers. Choose a “focal point" plant that is approximately 1-1/2 times taller than the height of your container when it is full grown. For example, if your pot is only 6" tall, then you wouldn't want a plant in it more than 9" tall when it's full grown or the container will look top heavy.
I prefer to pre-moisten my potting soil before adding it to my containers, especially if the mix is heavy in peat moss. Peat moss can be difficult to hydrate so it could take some time. One way is to add warm water to the potting mix as the peat moss absorbs warm water faster than cold water. Either way stirring the water into the potting mix will help it to absorb faster. Add water until the soil begins to stick together when you squeeze it in your hand.
I used to use these water storing crystalsin my potting mixes, but have found that adding more compost to my mix also works well and adds nutrients to the soil. These crystals will absorb water and then release it into the soil when the soil dries out, thereby allowing you to, theoretically, water less. When using these crystals I found I still had to water most everyday, but when you work all day these crystals can help your plants to be less stressed between waterings.
It's important to soak these crystals in water before adding them to the potting mix, otherwise they will expand while in the pot and make a huge mess. Some potting mixes already contain these crystals, which is another good reason why you should always pre-moisten your potting mixes. Complete instructions can be found on the package and they are available at most home centers and good nurseries, if you decide to try them. They do give you a little bit of a safety net in case you forget to water, but don't rely on them completely during long hot summer days.