by Andrea Laine, EMGV
Containers enable practically anyone to cultivate a garden. Containers can be placed inside or outdoors. Annuals or perennials, flowers or vegetables can be grown in containers. Herbs are especially well-suited for containers, and even trees and shrubs can be contained. Success lies in matching a plant’s growth requirements and growing conditions (Right Plant, Right Place) to the appropriate container and following good planting practices. Here is a primer on best practices for container gardening outdoors.
Choose plants with a confined or compact growth habit. Most annuals fit this description as well as nearly all leafy green vegetables and some fruit trees – apples, peaches and figs. If perennials are your choice, look for ones labeled “bush,” “dwarf,” “miniature,” or “specifically bred for containers.”
Container Depth and Diameter
Match the size of the container to the plant’s growth requirements. You want to give the roots room to spread and take hold.
- Annuals have shallow roots so a 6- to 8-inch depth container will do.
- Perennials or plants with a taproot (carrots) require greater depth — 10 to 12 inches.
- Herbs can thrive in 4- to 6-inch depth.
- Shrubs and trees need a depth and diameter of 12 inches each.
The number of plants you intend to include in one container and their size at maturity are factors to consider when choosing the diameter of a container. In general, larger containers do not dry out as quickly and are less likely to restrict growth, flowering and fruiting.
Be mindful of the porosity of the container you choose.
- Nonporous: plastic, metal, fiberglass, glazed
- Semi-porous: wood, pressed fiber
- Porous: clay, unglazed ceramic, terracotta
Porous containers like terracotta lose moisture more readily. They are ideal for growing plants that prefer drier conditions like cacti or succulents. Nonporous materials will hold moisture in, thus require less frequent watering. Unglazed ceramic is not recommended for a winter garden it is capable of absorbing water which under freezing conditions may cause the pot to crack.
Holes are necessary in ALL containers! This is a conundrum for those of us who are drawn to the many attractive containers on the market that do not offer drainage. Once upon a time, people would add stone or gravel to the bottom of the container to create drainage. The experts now recommend against doing that because it causes water to collect in the potting mix just above the gravel. Only when no air space is left in the potting mix will the water drain into the gravel below. In the meantime, plant roots are standing in water and developing root rot. Double-potting is the solution. Put the plant in a pot with drainage holes and place it inside the pot without holes. Remove the pot to water and drain.
The color of the container also comes with consequences. Dark-colored containers will absorb heat. This is a plus in winter, but could be a drawback in summer as the potting mix will heat up and dry out. Avoid placing dark- colored containers in full sun. Metal containers also get very hot in the sun as do blacktop driveways. Containers placed on black-top surfaces may need extra care.
Do not use soil from your yard! Purchase a quality potting mix or medium made especially for the type of gardening you will do. There are commercial mixes that are customized for indoor gardening, outdoor gardening, vegetables, cacti, African violets, orchids, etc. These mixes will have the right balance of fine and coarse textured components for what you want to grow. Plus there will be no weed seeds, disease or insects.
5-Steps to Planting Success
You’ve chosen a container, plant(s) and potting mix; Now it’s time to pull it all together and assemble your container garden. Aim for the soil line to be a quarter- to 3-inches from the top of the container. Plant the crown at the soil line.
- Fill container one-third with lightly damp potting mix.
- Remove plant(s) from its nursery container. Gently splay roots. Center plant in pot.
- Fill potting mix around the roots and up to the crown. Spread potting mix evenly. Do not tamp down the potting mix.
- Water well – until water drips from the container’s bottom. This initial watering will settle the potting mix. Add more potting mix if a water well forms.
- Top dress with mulch to reduce moisture loss. Be sure to keep mulch a few inches clear of the plant stem so air can circulate. Scatter pine cones or the spiky fruit of a sweetgum tree on top to discourage critters from digging.
Now sit back and watch your garden grow. Don’t forget to water regularly, fertilize periodically, and always site the container according to the plant’s light and temperature needs.
Master Gardener Handbook: Plants Grown in Containers
Container Gardening Planting Calendar for Edibles