Growing food

Containers have limitations in depth and volume – constraints that affect the air and water relationships in the growing medium and the type of plants you grow successfully. Larger containers, in depth and volume, enables larger plants to be grown. This is particularly important for growing woody plants like fruit trees in containers. Increasing soil depth has been shown to improve both growth and overall performance of these types of plants in containers. And while it’s possible to grow ‘large’ plants in smaller containers, the effects of this reduction in root volume on some plants can be severe, including reduced growth and overall size, much as we see with Bonsai plants. Of course, increasing container size also increases the overall mass, making them heavier and more difficult to move. 

Materials used to make containers can include plastics (esp. moulded products), ceramics, fiberglass, metal (especially powder coated or galvanized) and timber products. Containers should be durable and stable, able to last for many years of use. 

Where large volumes of media are held, such as fixed containers (i.e. vertical greening, podium planters, etc.), make sure the containers selected have sufficient side-wall strength for both the mass of the growing media and the growing plants. Choosing lightweight growing media can help to reduce weight loading in these situations, but a high-quality container is also important. 

If you are selecting plastic containers ensure that they are UV stabilized so they won’t degrade rapidly under outdoor light. Some cheap containers can break and fall apart after just a couple of years, rendering them useless for any further growing. 

Timber products can be great, but some caution is needed in their use, especially for food growing. Many outdoor timber products are CCA treated (copper chromium arsenic), containing chemical preservatives to slow decay and rot. Some research has shown chemical leaching into adjoining soils from this timber, and while the risks may be miniscule, in terms of food plant consumption, some caution is needed. If you are using these products outdoors, it’s best to line any CCA-timber planters with plastic internally to prevent plant root contact with the timber. 

Some ceramic containers, such as terracotta, can also be problematic. Fired ceramic products like terracotta are porous and absorb water readily, meaning they tend to dry out more rapidly as water is lost. This means they can dry out more readily than plastic pots of a similar size. Plant roots can also become fixed to the side walls of the container as the plants grow, making repotting difficult. Modern terracotta pots are mostly lined with a sealant to assist moisture retention and reduce water loss, but if not present, then lining with a plastic insert can overcome these problems. 

The position in which your containers are located is also important when growing food plants. Some special issues to consider with container food growing include: 

1. Light is always an issue to consider when growing food plants, both the total light (intensity) and seasonal light (daylength). An ideal growing space will be one that gets between 6-8 hrs of direct sunlight each day. Assess the orientation of your site and be aware of any shadowing or shading that might be present, particularly seasonally. In many urban locations building shadowing can reduce light intensity and daylength at different times of the year, especially in lower levels of a building where light is already compromised. The difference in light intensity between a north-facing to a south-facing balcony can be significant. Where low light is a problem, you can still grow food in containers, you just need to choose plants that can tolerate shade. More details on plant suitability for shade is outlined below.
    
2. Another feature of many urban sites is wind, especially at height. While the direction of seasonal winds is important to know, wind also changes with height. Wind gradients (i.e. change in wind speed) typically increases by about 1% with every 10 m of elevation. This may seem small, but when coupled with the upward force of wind on the exterior of a building, the ‘wind shear’ effects on plants on upper building balconies can be severe. This explains why rooftops and building facades at height can be very windy places. Wind also has an ‘eddy’ effect on the edge of tall buildings – meaning the closer you are to the edge, the gustier the wind will be, so avoid having plants too close to the perimeter of a balcony. If you have these conditions then some wind protection may be needed, such as physical barriers and non-permeable balustrades, to reduce wind exposure – benefiting both growing plants and the people who tend them!
    
3. Elevated heat can also be an issue in container growing, particularly on building rooftops and exteriors. This increased solar radiation means that container plants exposed to these conditions will need more irrigation but may also be need some form of shading to prevent leaf scorch if temperatures are extreme. Another consideration in these sites is to use lighter-coloured containers, rather than darker-coloured ones. Black plastic pots readily absorb heat and can be as much as 20 ° C warmer in their affected root zones compared to white pots, sometimes leading to lethal root temperatures inside the container.