In subtropical climates, autumn and winter are great for growing vegetables as the temperature settles down to between 5 and 20 °C until February. So, if you haven't planted anything yet, there is still time to grow some of your favorite veggies, and here are some important key elements to remember for an easy growing edible rooftop garden.
1. Understand why temperature matters
Plants are sensitive to temperatures in the soil and in the air.
When you start from seed, it is important to first check the germination temperature of the plants you have chosen to grow. If the soil is too hot or too cold, the seeds won't germinate. The information can be found easily on the Internet but is also usually written on the back of every seed bag.
Now the real question is how to take the soil temperature? Well, you could use a soil thermometer, but a cooking thermometer will do just fine, too. A thermometer is particularly useful if you plant seeds directly into the ground in areas where temperatures can go below 0 °C. However in subtropical climates, the soil temperature usually suits a large number of plant germination temperatures.
That said, experienced gardeners will often first plant seeds indoors or in a greenhouse in order to protect the seedlings from insects or temperature changes. The air temperature is also very important for plant growth and development along with air humidity, CO2, water, sunlight and nutrients. Ideally, all these factors should be in balance to reach the optimal growth of plants. In the case of extreme temperatures, the photosynthesis process will not work efficiently, which can lead to plant death. The temperature limit depends on the plant variety and varies regardless of light intensity. However, in medium temperatures, between 10 - 20 °C, the photosynthesis process works well. Look for the optimum temperature for the photosynthesis process in the plant you would like to grow if you want to be sure when the best time to plant is.
Also, remember that every plant needs a specific quantity of each factor (temperature, air humidity, CO2, water, sunlight and nutrients) but their balance can be adjusted according to the growing location characteristics, in other words your rooftop or balcony. Another way to see it is that plants are alive and can adapt to their environment. To adapt, they will need more of certain things and less of others: more or less water, more or less sun etc. For example, mint can grow well in full sun as well as in part shade. However, if mint is planted in part shade environments, it will need less water and vice versa. Therefore, it is harder to give a clear and unique tip besides recommending that you observe both short and long term how your plants react to the environment in which you chose to grow them.
2. Acknowledge your garden sun exposure
Now, let me tell you a story...
"Once upon a time, there was a cute sun loving plant which was one day chosen for its beauty among all the other green babies. It was placed in a corner to become the show stopper of the realm, the sunlight could reach it no more and so it started to despair. First, it did not grow new leaves. Then the insects became a deadly threat as well as other plant diseases. It was noticed that Belle needed help, so it was given more water thinking that was the real problem and yet, instead the bell was rung for the cute sun loving plant's end, leaving it drowning in love when what it really longed for was the sun."
Am I being too dramatic? Sadly, this is the real story of a lots of indoor and outdoor plants. The opposite situation also occurs when a shade loving plant is placed in a too sunny spot. This is why designing carefully your garden at first is so important.
Water and nutrients are the two elements that can be most easily adjusted according to your plant's needs in a chosen location. For example, the soil composition can be adapted so it retains more or less moisture while compost or fertilizer can be added to the soil on a regular basis in order to provide the nutrients that your plants may need.
However, the sun exposure that your location gets is not as manageable especially in the case of insufficient sunlight. Indeed, unless you are using artificial lights to compensate for a lack of sunlight, your plant will likely die. By planting sun loving shrubs or small trees in strategic places, shade can be provided for other plants. Growing vines or climbing plants or even placing a light piece of fabric above sun sensitive plants can also help to filter the sunlight as it acts as a canopy.
How much sunlight?
The sun exposure is usually categorized into 3 different types that you may have already heard of. The illustration below explains it quite clearly. Be careful though, those three terms refer to the amount of direct sunlight your plants get.
In addition to this information, please note that morning sun tends to be less aggressive than afternoon sun so if your location is getting more direct sunlight in the morning, it will be more suitable for fragile edible plants like lettuce. If there is no direct sunlight at all where you want to grow edible plants, do not give up! Check if there is any sun reflection coming from the buildings' windows surrounding your rooftop. If the reflection is reaching an area where you can grow plants, it can be counted as direct sunlight.
Full shade has a different meaning for indoor and outdoor plants. For outdoor plants, it means that plants can "see the sky" but cannot get more than an hour of direct sunlight per day. Although you might not be able to grow tomatoes and basil, there is still a possibility to grow some vegetables!
3. Grow edible plants, not vegetables
When we want to start growing vegetables, we often have a very specific idea of what we want to grow. As we saw above, this ideal may have to be reconsidered if the sun exposure of your location is not suitable for the vegetables you had in mind. You might feel disappointed at first but here is something you can cheerful about: There must be edible plants that can be grown in your location. Vegetables are actually edible plants but too often, the picture of veggies we have in mind is narrowed down to what we can find in supermarkets. They represent just a tiny drop of all what nature is offering us to eat.
Based on how many hours of sunlight there is on your rooftop and the average temperature, you can start making a list of all the vegetables (or should I say edible plants) that you can/want to grow. Look for native edible plants or perennials or even heirloom varieties which would be more suitable for your location. Choosing more suitable varieties will not only lead you to a promisingly successful harvest but also a whole new way to cook and eat as you will adapt your recipes or even create new ones according to what you have harvested. It is a creative win-win in both kitchen and garden.
4. Containers matter
The last important factor, especially for rooftop gardening, is the size of the pot. Plants grow roots to reach as much nutrients as possible. The container dimension has to be chosen according to the plant root system. In the case of a too small container, the roots will start growing intensely on the side and end up in a circle at the bottom. This is what we call root-bound, and it is easily diagnosed after taking the plant out from its pot. As for germination and photosynthesis temperatures and also for sunlight requirements, the size of the container will vary depending on the type of plants you choose to grow. If you grow from seeds, it is better to repot your seedlings in a final size container than in an "in between" size container.
You may also have noticed that seedlings or young plants purchased in nurseries can show signs of root-bounding which is logical as nurseries use tiny individual pots for both space and water saving purposes. If so, before repotting, delicately help the roots to straighten, this will help them spread through the entire space of the container. Also, if some roots are way longer than others, it is preferable to trim them off so the root-ball gets back to a unified shape and helps stimulate root growth more evenly. In case of severe root-bounding, root pruning is the final option.