Is your garden in the sun? …or the importance of light for growing food

If you ever have propagated seeds on the windowsill, you will have experienced how small seedlings bend towards the light and often become leggy due to insufficient light. It might seem obvious, but light is one of the most important elements in growing food. And given that we live in Ireland, in North Western Europe, we can never have enough of it. The source of light is the sun and direct sunlight is best for plants.

Photosynthesis is the foundation of who we are and the lives we can lead

But how does it work, how do plants use sunlight to grow and produce food? They eat or in biological terms photosynthesise: By taking in water (H2O) through the roots, carbon dioxide (CO2) from the air and light energy from the Sun, plants can perform photosynthesis to make glucose (sugars) and oxygen (O2). The natural pigment chlorophyll, that gives plants their green colour, is the vital catalyst to absorb the sunlight.

There are not many gardening books where the author explains photosynthesis in the first chapter like Marie-Luise Kreuter in her seminal work “Der Biogarten” and its only through her I came across Brendan Lehane and his book “The Power of Plants”, published in 1977 (!). His description of the process is truly illuminating:

“If one eats a freshly harvested leaf at daylight, one eats something that was part of the sun only eight minutes ago, the time sunlight needs to reach the earth. The conversion of solar energy into food takes a fraction of a second. This process constitutes the essential key to all life on earth.”

The oxygen we breathe is largely produced through photosynthesis, which is responsible for maintaining the oxygen content of the Earth's atmosphere and supplies most of the energy necessary for life on Earth. Since we cannot live without food and oxygen, the impact of photosynthesis on our daily life is crucial.

How much sunlight do vegetables need?

So going back to our garden and how much sun is needed for successful growing. The majority of our garden plants require full daylight all day long for the best outcome, but there are some vegetables and herbs which are more tolerant to partial shade. Root crops and leafy vegetables are more accepting of less light conditions than the sun-hungry tomatoes. You might notice that shade enduring plants frequently have much bigger leaves to make best use of reduced light levels. Here are some minimum requirements of light, ideally in the form of direct sunlight:

Fruiting vegetables like tomatoes, peppers, aubergines, cucumbers, melon and squash need at least 8 hours of sun per day.
Root vegetables like carrots, beetroot and radishes can do with 6 hours of sun per day.
Leafy vegetables like lettuce, spinach and kale can survive on 4 hours sunlight.
Herbs like mint, parsley, sage, oregano and thyme will still need about 6 hours of sunshine per day.

Bear in mind though, that these times are minimums and increasing the amount of sunlight for our vegetables will result in better yields, quality and flavour. But although our gardens should be in full sun, it is heartening to know that plants will adapt and overcast days, a shady garden or partial shade during the day isn’t the end. Just make the most of what you have!

What matters to photosynthetic activity and what we can do

As the photosynthesis equation shows the sun delivers the energy, but plants also need carbon dioxide, water and a healthy soil for growth and production of food. Here we can help and our actions as gardeners, growers and farmers can be vital in the fight against global warming, habitat loss, soil erosion and air pollution.

The important message here for us gardeners is that microbes matter to photosynthetic activity throughout a plant’s life. We know that plants push carbon into the soil through root exudates, that microbes consume that carbon, then respire much of it out in the form of CO2, that plants need for growing. We also know that in healthy soil with high organic matter soil microbes respire quite a lot of carbon dioxide. So keeping soil fertile with regular additions of compost not only helps generating high carbon dioxide levels, but also helps in keeping the plant healthy and productive. And because we humans exhale carbon dioxide, remind yourself the next time you’re pruning tomatoes in your polytunnel or greenhouse, that you’re also helping to fuel the carbon cycle!

In all of my polytunnel courses I ask participants about the best time to water, ideally in the morning, when the sun is out and plants wake up to photosynthesize and too little water can have severe effects on a plant’s ability to carry out the process. Mind you, too much water and the ability to absorb oxygen and nutrients from the soil are hampered too. And again a soil rich in all essential nutrients is key for plant growth. For example, the chlorophyll molecule, which is an essential component of photosynthesis, contains magnesium atoms. So if magnesium is lacking in the soil, plants may not be able to make enough chlorophyll to carry on optimal levels of photosynthesis.

Gardeners as attentive apprentices of nature

Spring is here and the new growing season is before us and when out in the garden sowing and planting we might reflect on Marie-Luise Kreuter’s closing sentences on the chapter about photosynthesis:

“A gardener who, in his green paradise, not only looks at the shiny surface of his salad leaves, but also sees through them and grasps what is happening under the skin, such a gardener … remains an attentive apprentice of nature throughout his life.”