What is the Magic Amount of Sunlight to Grow Maximum Veggies?
Most gardeners don’t know the facts.
It’s true. I didn’t understand the various combinations of day length and sunlight intensity myself for many years. When I understood these facts, I figured out why my cauliflower only grew to the size of a baseball, and my onions bloomed instead of forming a bulb.
Very few people understand precisely how much light vegetables need.
A friend asked me to come over so they could show me the proposed spot for their new vegetable patch. “Right here, under the big oak tree in the backyard. The plants won’t be too hot and will get all this shade in the afternoon,” they told me.
Sorry. This won’t work. I know we see grass growing under trees and flowers in partial shade, but food-producing plants require more light.
The sun is an essential source of energy and growth for vegetable plants. Too little sunlight causes light green foliage, lack of production, and typically lanky stalks as the plants try to access more light. Weak plants are more susceptible to disease and pests.
In addition to the intensity of the sunlight, we must also consider the length of the days, and even the angle of the sun, when planning our veggie patch. Let’s look at each of these items.
Intensity refers to how many hours of sunlight between 10 am and 4 pm.
As a general rule, plants with edible roots and leaves can tolerate more shade than those that produce edible fruits. However, even these will grow more slowly with less sun.
Seed packages often say plant in full sunlight, but what does that mean?
Full sun = no shade for at least six to eight hours.
Partial shade = the filtered shade under a high tree, less than six hours direct sun
Full shade = this area gets no direct sun or less than two hours.
Edible roots include carrots, potatoes, turnips, radishes, and more. We eat the leaves of lettuce, spinach, chard, kale, and much more. These vegetables can tolerate partial shade but do need at least four hours of sunlight each day.
The fruiting vegetables include tomatoes, peppers, eggplants, and squash. We also consume the buds of what would be the flowers of some plants such as broccoli, Brussels sprouts, and cauliflower.
Understand how darkness and daylight affect plants.
The sensitivity of plants and animals to the daylight length or, more accurately, the length of darkness, is known as photoperiodism. Many flowering plants sense changes in the length of darkness, and it triggers them to bloom.
This is why your radishes may suddenly send up a flower shoot and stop growing the radish on the bottom — the lengthening spring day length told the plant to start producing seeds and stop making radishes.
Remember that nearly all vegetables bloom even if we only eat the roots.
For instance, we eat the carrot root, but the plant still must flower to produce seed.
Vegetables can be divided into these categories:
Long-day (LD) sensitive plants bloom when the hours of daylight exceed 12 hours, and darkness is less than 12.
This category includes beets, carrots, fennel, lettuce, potatoes, radishes, spinach, Swiss chard, and turnips, and more.
Short-day (SD) sensitive plants flower when they get less than 12 hours of daylight and the darkness exceeds 12 hours.
In this group, you won’t find many vegetables, but it does include cannabis, soybeans, and cotton.
Daylength neutral plants aren’t stimulated to flower by day length. These plants include cucumbers, tomatoes, peas, sunflowers, Brussels sprouts, cabbage, corn, and kale.
Onions have their own set of rules. They are divided up by long-day varieties, which are triggered to begin bulb development by 14–16 hours of light, short-day that start bulbing at 11–12 hours of day length, and intermediate varieties which bulb at 12–14 hours of light.
There are additional factors that trigger reproduction, including the plant’s age, development stage, and vernalization.
Understand Persephone Days
The length of your days and nights depends on your latitude or distance from the equator. Find your longitude and latitude, and you can use this tool or this one to determine the daylight length where you live.
Plants will become dormant when the daylight hours dip below ten. Eliot Coleman, the author of The Winter Harvest Handbook, calls this short light time of year the “Persephone Days”. She was the all-powerful goddess of nature, responsible for both the growth and the death of plants.
Here in Central Texas, on the shortest day of the year, we still have ten hours and 12 minutes of light, and yet vegetable growth becomes imperceptible during the middle of winter. For comparison, the northern city of Augusta, Maine, will only get 10 hours and 12 minutes on Halloween, October 31.
The angle of the sun affects plant growth.
Also, the low angle of the sun in the winter months results in less photosynthesis than the same amount of daylight when the sun is directly overhead.
In the summer, the direct sun can be too intense and require shade to reduce the amount of sunlight by up to 30% or 40%.
author’s photo of 30% sunlight filtering cloth in Texas summer
The goal of all cultivation is to provide the optimum conditions for healthy plant growth. Yes, there is a lot to learn, but it makes gardening fascinating.
Experiment in your garden, tinker with where you grow different crops. Keep notes of when and where you plant things, and you’ll add to your knowledge year by year.
Have fun, and get growing!