Container Gardening: A Toronto Master Gardeners Guide

What is Container Growing?

Not all gardening needs to be done directly in the ground. There are many reasons to grow flowers and/or vegetables in containers or planters. They are being used by condominium dwellers to grow food and flowers on their balconies; by gardeners who want to use smaller, sunny areas like driveways; by those who are growing older and don’t enjoy being on their knees and bending over; by those in wheelchairs who are more comfortable with a raised bed; by those who have roof gardens; and by those who want a glorious display of flower pots for their entrance.

Containers

Containers and planters can come in any size and any shape. Bigger plants require bigger containers. Containers can be made out of anything from heavy textile, wood, plastic, metal, ceramic or plastic. There is a very wide range of containers that can be bought, others that can be foraged (old aluminum bathtubs make great containers if you put in drainage holes), and there’s a lot of information online on how to build your own. The most important things to remember are that the container should have holes that provide good drainage, the soil you use should be light and fertile (have a lot of compost or vermiculite in the mix), you should keep your plants well watered, and have good air circulation (raising the containers off the ground helps).

Soil and Fertilizer for Containers 

The soil that you use in containers or planters does not have the advantages of a garden bed, which has a rich life of micro- and macro-organisms including our invaluable earthworms that make sure the soil is well aerated. Therefore, make sure that you use a light potting soil. A combination and balance of top soil, vermiculite and compost is ideal. A reasonable guide is 50% soil; 50% organic matter. Container soil usually needs replacing each year, but if your container is large, replace at least a third of the container with fresh soil.

Potting soil can be purchased already formulated from gardening stores. If you would like to make up your own formula, the following links have useful information on potting media:

• Do it yourself. Creating the Right Soil Mix for Square Foot Gardening.     https://www.doityourself.com/stry/creating-the-right-soil-mix-for-square-foot-gardening
• University of New Hampshire Extension. What is the best way to grow potatoes in containers?    https://extension.unh.edu/blog/2020/03/what-best-way-grow-potatoes-containers

Whether you need fertilizer will depend on the freshness of your soil and what you are growing. The following links have useful information on container fertilization:

• University of Georgia Extension. Gardening in containers. https://extension.uga.edu/publications/detail.html?number=C787
• Clemson College of Agriculture, Forestry & Life Sciences. Container vegetable gardening. https://hgic.clemson.edu/factsheet/container-vegetable-gardening/

When filling the planters with soil, do not add any other material at the bottom of the container. People often mistakenly put rocks, gravel or broken ceramic and other materials to help improve drainage. Because of osmosis, this ends up having the opposite effect and hinders drainage leaving a very wet layer than can rot around the roots.  Dr. Linda Chalker-Scott, a horticultural professor and one of the contributors to The Garden Professors, explains in her article The Myth of Drainage Material in Container Plantings that water does not move easily from fine textured material (soil) to a coarser material (gravel).

Sunlight

As with growing in garden soil, flowers and vegetables grown in containers have different light requirements. To grow most vegetables, you need more than six hours of direct sun, but greens and peas and some beans, and many herbs, need less. Most annual flowers can be grown in containers, and their light requirements differ.

One of the major advantages of containers is that they can be moved. So as the sun moves during the season, move your containers so that they get the best light. Colorado State  University  has a guide on Container Gardening.

What to grow

Growing Annuals in Containers

You can have a glorious display of flowers grown in containers. Annual flowers can be grown very easily in containers from seedlings, or by direct seeding. Most flowers usually require at least six hours of sun each day to thrive, but you can select for flowers that do like the shade. Toronto Master Gardeners has a gardening guide on growing annual flowers in the shade for your reference:

https://www.torontomastergardeners.ca/gardeningguides/toronto-master-gardeners-guide-to-annuals-for-shade/

Growing a combination of flowers and vegetables in your containers can be very rewarding. The flowers will attract pollinators to your vegetables, and vegetables are very attractive in their own right, and of course, will feed your family through the summer and fall. If you are growing indeterminate tomatoes  (vine style tomatoes, branches and side shoots continue to grow even after fruit has set) or any of the vines, you will need to consider supports for your plants. Tomato cages of many sizes work well.

The urban farming movement across Canada has now made growing vegetables in containers very popular. They have taken over unused space in schools, places of worship, hospitals, and even parking lots.  They often have a strong social action focus and the work they are doing is admirable. Their websites are a mine of information on both how to grow in containers and how to be part of their sustainable-food movement. You can learn more about urban growing in Toronto from this link:  http://torontourbangrowers.org/

Growing Perennials in Containers 

Growing perennials in containers has become a very popular trend. Before choosing your perennials, there are a few important factors to consider. The following information comes from a number of the Toronto Master Gardeners archived posts:

Overwintering perennials in containers is one of the biggest challenges faced by container gardeners in our zone.  The freeze-thaw cycle is the main problem; that is, the melting of the water in the container’s soil during sunny or warmer spells, followed by freezing when the temperatures dip again.  This is what kills a plant’s roots over the winter.  Your most important starting point is the container itself: it should be as large as possible (the more soil it can contain, the more insulation it will provide).  Ensure that the containers are freeze-thaw resistant.  Ceramic and clay pots will probably crack as will cheaper plastic pots. During the winter months, shrubs often suffer from moisture loss (desiccation) if they can not pick up enough moisture, especially before dormancy and freeze. Wind protection will also help minimize moisture loss.

For your container you may wish to consider a mixture of evergreens and perennials. Most perennials do not flower for the entire season. Incorporating perennials with height variation that flower at various times and have interesting foliage along with some evergreens will keep the container interesting throughout the year.

The warm-up period is what is most damaging to perennials in containers. To help them overwinter, try to keep things evenly frozen or cold. Also, keep them in a shady corner and in insulated pots (either inside, if they are big enough or outside if they are not).

Garden Making has an excellent article on how to overwinter perennials in pots: How to Overwinter Perennials in Pots

Container Gardening with native plants has become increasingly popular. According to the National Wildlife Federation “A plant is considered native if it has occurred naturally in a particular region, ecosystem, or habitat without human introduction.”

Native plants have formed symbiotic relationships with native wildlife over thousands of years, and therefore offer the most sustainable habitat to pollinators. The Missouri Botanical Garden, Container Gardening with native Plants illustrates container designs for various light conditions.

Best evergreens for containers

The best trees for containers are small and slow growing with compact root systems. Evergreens that are cold hardy in Toronto (Toronto’s hardiness zone is 6) will need to be rated at zones 3 or 4 when in a container. Dwarf cypress appears to be a questionable survivor in our climate, however, Chamaecyparis or False Cypress would be a suitable choice and is hardy to zones 4 and 5 and there is a great variety of colour, foliage and form that is available. Because evergreens transpire throughout the winter, they must be kept watered right up until the soil ball freezes hard. Spread out the roots at potting up for all plants. The Brooklyn Botanic Garden article  Dwarf Conifers in Containers: Designing a Minature Landscape gives additional information.

Grasses

The use of a grass like golden variegated Japanese Forest Grass can offer a fine flowing texture with colour interest as well. Japanese Forest Grass (Hakonechloa) comes in a variety of foliage colours, ranging from from ‘golden variegated’ to ‘all gold’. This plant can grow in full sun, part shade and full shade. It has a beautiful cascading growth habit.

Ground covers

Some other ideas you might want to consider include the use of ground covers that can cascade or flow over the edges of your containers.  Creeping Jenny (Lysimachia) comes in a lime green or medium green and has a yellow flower.  Lamium can also provide a nice variegated cascade affect with either purple or pink flowers. Both of these plants grow in part shade-full sun. Some of the Sedums can cascade as well.  Have a look at yellow-flowered Sedum  sexangulare, or even  hens and chicks (Sempervivum spp. and cvs.) for the sunny side of your planter.

Other references you may enjoy include this one that has good information about what herbs to grow in containers: https://www.durhammastergardeners.ca/resources/Planting-Herbs-in-a-Container-handout.pdf

This reference has some great design ideas and suggestions on what to grow: https://www.emmitsburg.net/gardens/articles/adams/2004/container_gardening.htm

 

References

Colorado State University Extension. Container gardens. https://extension.colostate.edu/topic-areas/yard-garden/container-gardens-7-238/

Toronto Master Gardeners. Annuals for shade: A Toronto Master Gardeners Guide. https://www.torontomastergardeners.ca/gardeningguides/toronto-master-gardeners-guide-to-annuals-for-shade/

Toronto Urban Growers. http://torontourbangrowers.org

Gardenmaking. How to overwinter perennials in pots. https://gardenmaking.com/overwinter-perennials-in-pots/

National Wildlife Federation. Native plants.  http://nwf.org/Garden-for-Wildlife/About/Native-Plants

Missouri Botanical Garden. Container gardening with native plants. https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/visual-guides/container-gardening-with-native-plants.aspx

Brooklyn Botanic Garden. Dwarf Conifers in Containers: Designing a Miniature Landscape https://www.bbg.org/gardening/article/dwarf_conifers_in_containers

The Spruce. How to plant Golden Japanese Forest Grass (Hakone Grass).  https://www.thespruce.com/golden-hakone-grass-2132482

Fine Gardening. Golden creeping Jenny Lysimachia nummularia ‘Aurea’.  https://www.finegardening.com/plant/golden-creeping-jenny-lysimachia-nummularia-aurea

Wisconsin Horticulture Division of Extension. Spotted deadnettle, Lamium maculatum.  https://hort.extension.wisc.edu/articles/spotted-deadnettle-lamium-maculatum/

Missouri Botanical Garden. Sedum sexangulare.  http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=279397

The Spruce. How to grow and care for hens and chicks (house leek). https://www.thespruce.com/how-to-care-for-hens-and-chicks-plants-2132609

Durham Master Gardeners.  Planting herbs in a container. https://www.durhammastergardeners.ca/resources/Planting-Herbs-in-a-Container-handout.pdf

The Master Gardeners. Container and small space gardening. https://www.emmitsburg.net/gardens/articles/adams/2004/container_gardening.htm

University of Georgia Extension. Gardening in Containers. https://extension.uga.edu/publications/detail.html?number=C787

April 20, 2020. Ask a Master Gardener. Planning a container garden.  https://www.torontomastergardeners.ca/askagardener/planing-container-garden/

May 12, 2020. Ask a Master Gardener. Perennials for a container.  https://www.torontomastergardeners.ca/wp-admin/post.php?post=26356&action=edit

July 2, 2007.Ask a Master Gardener. Soil for container gardening?  https://www.torontomastergardeners.ca/wp-admin/post.php?post=18177&action=edit

 

Date Prepared: February 2022