How Much Sun Do Tomato Plants Need at Different Stages of Growth?
Tomato plants grow very quickly, doubling in mass approximately every two weeks under favorable conditions. To keep up with this high growth rate, they need a lot of energy that is produced through photosynthesis in the presence of sunlight, water, and nutrients. The big question is, therefore, how much sun do tomatoes need? Typically, tomato plants require anywhere between six to eight hours of sunlight a day, and these need to be under full sun.
It's not as simple as that though, read on to better understand sun exposure and what your tomato plants need. This is one of the most overlooked issues when it comes to tomato plant care.
Different Stages of Sunlight: Which Is Better?
Before looking into the different stages of sunlight and what it pertains to, you should know that tomato plants need high-light intensity and not high-heat intensity. Additionally, the intensity of the sun fluctuates a lot during the day based on the position of the sun, cloud cover, etc.
When planting tomato plants, the goal is to maximize light while minimizing any harmful effects of heat. It is common to categorize sunlight by the time of day.
For the process of photosynthesis, chlorophyll (the green part of the plants) absorbs ultraviolet (UV) light, which is at its highest intensity during the early hours of the day accompanied by minimal heat.
Midday Sunlight (11 a.m. - 2 p.m.)
This is the time of the day where the sun is at its highest position, and tomato plants get a lot of direct sunlight which they love. Unfortunately, this increased light comes along with increased heat. It is, therefore, a good idea to use a directly above your tomato plants if you live in a region where the direct heat can cause temperatures to rise above 90°F (32°C) during the day. Alternatively, some people strategically grow sunflowers to provide shade from the direct midday sun. shade cloth
The afternoon and evening sunlight provides the necessary UV radiation without the additional heat. To allow your plants to get access to this sunlight, you should remove anything that blocks light from the west.
The main reason why the morning sun is better than the afternoon sun is that it helps evaporate any dew on the leaves. Moisture on the leaves overnight can lead to fungal infections. Additionally, the slow increase in UV radiation and heat helps acclimate the plant to the new day when compared to the plant receiving a sudden burst of high-intensity midday sun if it were planted near a west well (no sunlight from the east).
Which Is Best?
Provided that the plant is not being stressed by heat, there is no real difference other than the slight benefits of the morning sun as explained above. As long as your plant gets at least six hours of direct sunlight, with eight being perfect for most stages of growth, it does not matter at what time the plant gets the sun and it does not have to be continuous either.
Pro Tip: When possible, it is advisable to plant your tomato plants in a north-south orientation, because this allows them to get the early morning sunlight from the east without casting a shadow on the other tomato plants in your garden.
Sunlight at Different Stages of Growth
The ideal amount of sunlight for tomato plants changes depending on the stage of growth. Of course, you cannot move a grounded plant around. But understanding this and the problems associated with excess sunlight could help you pick the right spot in your garden and prevent mistakes in caring for the plant.
Note: This section is only directly applicable to determinate varieties, because indeterminate tomatoes go through all these stages simultaneously. If you are growing indeterminate tomatoes, you should still read through, because it contains a lot of helpful information that applies constantly once the first flowers bloom.
This stage refers to the growing period after transplanting into the final growing location, either the ground or a container. Provided that the plant has sufficient nutrients (see my guide on fertilizing tomatoes), especially nitrogen, the plant mass would double every two weeks. During this stage, the plant needs a lot of energy, but not as much as the fruiting stage. Therefore, six hours of direct sunlight should suffice.
Flowers are very delicate and they could fall off without being pollinated if the temperatures go above 90°F (32°C). This stage also requires slightly more energy than the vegetative growth stage. But six hours would still suffice, even though seven hours of direct sunlight would be ideal. It is also important to make sure that your plant does not dry up, as the flowers are sensitive to water fluctuations. More about water stress caused by increased heat can be found further down this article.
This is another delicate stage, as tomatoes are highly susceptible to sunscald if they are exposed to intense sunlight at high temperatures. At the same time, this is a very energy-intensive stage, and tomato plants could use up to nine hours of direct sunlight, with eight still being sufficient.
It is very important that no fruit is constantly exposed to the sun, and one way to do this is by being smart when you prune your plant. You should not prune too much and expose any existing or future fruit.
Once the fruit has grown to maturity, there is no additional need for nutrients and energy. Ripening takes place through a hormone that releases a certain gas, ethylene.
Once you notice your fruit beginning to ripen, it is wise to cut it off the plant and let it continue to ripen on your kitchen counter. In addition to saving the fruit from animals, it can help prevent the tomato cracking on the vine due to a sudden influx of water.
Issues With Too Much Sun
Being tropical plants, they do well under temperatures of 90°F (32°C) in the day and under 75°F (24°C) in the night. Anything above these temperatures results in stress. A common question asked is whether tomatoes can get too much sun. There's no such thing as too much sun, provided the plant receives sufficient water and is in a stable ecosystem, but when things are out of balance or when the flowers and fruit are not protected, too much sun can be detrimental.
As discussed above, one of the problems you could face is sunscald. It is an issue where the part of the fruit constantly exposed to the sun develops a region of discoloration. This region hardens and can in many cases begin to rot. As we now know, shade and proper pruning techniques help prevent this issue.
If you live in a region where temperatures can soar above the limits mentioned above, you should avoid growing varieties that have sparse foliage.
Damage to Flowers
As explained above, high heat can cause blossoms to drop before they are pollinated. Mulching and maintaining a good balance of water and nutrients will not prevent the flowers from falling, but it will help the plant bounce back with a new batch of flowers if the extreme temperatures do not last for many days.
At these high temperatures, fruit on the plant will not fall off. But they will not grow as fast, because the plant is now in survival mode.
High temperatures result in the plants releasing an increased amount of water through their stomata. They therefore need to absorb more water from the ground. However, without a good mulch or water supply to the plants, the foliage will begin to wilt. This is because the ground also loses a lot of water through evaporation, limiting the amount available to the plant.
Do not worry too much if you notice leaves wilting, even though your plants have access to water, as this is a natural defense mechanism. Your plants can, therefore, wilt due to a lack of water or as a self-protection mechanism.
Water stress can cause the fruit to stop developing and in some cases drop from the plant. It can even lead to blossom-end rot, where the bottom of the fruit turns black and mushy. Read my guide on watering tomatoes to understand how much and when it is best to water.
Growing Tomatoes in Low-Light Conditions
Tomatoes are sun-loving plants, and they need enough sun if you want them to grow well and produce fruit. They definitely will not die under low-light conditions, but they just won't be as productive. You can, therefore, in principle, grow tomatoes in the shade, but not in complete darkness.
There are quite a few things that you could do to help overcome problems of low natural sunlight. For starters, you could make the best use of the light that you already get. Paint walls white to help reflect light onto your plants grown next to them, use mirrors, etc.
Your other option would be to grow tomato varieties that do not require a lot of sunlight. Most varieties of cherry tomatoes, for example, can handle lower sun intensities than larger tomato varieties, though harvest may be smaller. According to Kendra on New Life on a Homestead, bush tomatoes that are adapted for colder regions or varieties that are ready for harvest in under two months are good options. She suggests the New York, Siberia, or San Francisco varieties. I personally do not have any experience with these.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.