Growing plants in pots and planters can add considerable stress, weight-wise, to your balcony or roof. It may seem an obvious consideration, but judging by what is common in many homes, it is clear that many people are simply unaware that their roof or balcony garden has exceeded the weight limit that the building codes stipulate for any given situation. It may be worth considering the consequences, both legal and otherwise, in the event of an accident.
It would be advisable therefore, to invite a building engineer to check your balcony. Meanwhile, here is a typical example. A balcony may have a weight limit of 150 kilograms per square meter. The volume of a very large pot containing a tree that reaches some 4 meters in height, could have a volume that is about 150 liters. As one liter of water weighs one kilogram, it follows that the weight of the pot filled with water, would reach 150 kilograms. This is without potting soil, and the weight of the tree. I have often seen the weight limit exceeded four or five times over! Here then are some guidelines to keep you on track.
The Container
Firstly, choose lightweight containers. Plastic is obviously the lightest material, although you may dislike, as I do, the very idea of mock terracotta pots. However, there are many plastic products available today that are highly aesthetic. The rule is to avoid products that are pretending to be something they are not, such as some fake ceramic container.
The size of the pot is the next consideration. To keep the weight down, you have to limit the dimensions of the container. This means that it is virtually impossible to grow plants beyond a height of 2-2.5 meters, (6-8 ft) because taller plants, require a deeper pot to prevent them toppling over in the wind. The deeper the pot of course, the greater the weight exerted on the floor.
The Potting Mixture
There are excellent horticultural reasons for growing container plants in an artificial potting medium as opposed to garden soil. However, just in case you were thinking of ignoring these, just think of the weight factor, as the weight of soil, especially clay soil, expands exponentially when saturated with water. From every point of view therefore, garden soil should never be used in containers grown on the roof or balcony. The question is which type of potting medium is the most suitable, at least from the weight angle? Unquestionably, the media such as “Perlite”, which are both extremely light, yet retain sufficient moisture, are to be preferred to the usual potting mixtures, whose weight also increases significantly, when wet.
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