Weekly Light

Understanding Weekly Light in the Piedmont

The amount of sunlight that reaches plants changes dramatically over the course of a year. In Raleigh and the broader Piedmont region, the difference between summer and winter light can exceed 25 mol m⁻² d⁻¹—a shift large enough to influence plant growth, flowering, dormancy, and winter survival.

The chart above shows an approximate Daily Light Integral (DLI) curve for Raleigh, North Carolina. DLI is a measurement of the total amount of photosynthetically active light (PAR) that reaches a one-meter area over a 24-hour period. It is the most useful way to describe the light plants actually receive.

Using weekly estimates keeps the tool simple, while still revealing the strong seasonal rhythm that drives plant behavior in the Piedmont.

Why Light Changes So Much Over the Year

Two factors shape the seasonal light curve:

1. Daylength
In summer, the sun is above the horizon longer. Winter days are short. This alone explains a large part of the DLI changes.

2. Solar elevation angle
The sun climbs much higher in the sky during summer months. A higher sun means more direct light reaching a horizontal leaf surface, and the midday peak becomes exceptionally bright for plants adapted to full sun.

When both factors combine, July offers more than twice as much usable light as December.

What Gardeners Can Learn from This Chart

Why houseplants struggle in winter

Even in a bright window, winter PPFD levels can be one-third of what the same window delivers in summer. Many tropical houseplants respond with:

• leaf drop

• elongated growth

• reduced flowering

• slower recovery from pests or repotting

Understanding the seasonal low points helps homeowners adjust expectations and caregiving practices.

Why cool-season crops thrive in fall and winter

Leafy greens and brassicas (lettuce, kale, collards, spinach) do very well with moderate DLI values and cool temperatures. Even “low sun” December conditions often provide enough light for steady growth, especially with south-facing placement.

Why fruiting crops peak in midsummer

Tomatoes, peppers, okra, cucumbers, and basil flourish when DLI exceeds 30–35 mol m⁻² d⁻¹. These species evolved in high-sun environments, and you can clearly see where the year provides enough light for maximum yield.

Why lawns grow fastest in late spring to midsummer

Warm-season turf species align with the steep rise in light from April through July. Cool-season lawns respond strongly to the fall climb just after the September light minimum.

Interpreting the Numbers Below the Chart

When you move the slider, you will see:

• Date: midpoint of the weekly interval

• DLI (mol m⁻² d⁻¹): total light the plants receive

• Daylength (hours): time between sunrise and sunset

• Average PPFD (µmol m⁻² s⁻¹): typical daytime intensity

• Max PPFD (µmol m⁻² s⁻¹): estimated peak brightness around solar noon

For high-sun species, Max PPFD is a helpful indicator of midday intensity. For shade-adapted species, DLI often matters more than the noon peak.

Notes on Accuracy and Interpretation

This chart uses a clear-sky solar model commonly used in agricultural and horticultural engineering. It does not attempt to show cloudy-day dips or local shading from trees, houses, or landscape features.

However, it accurately portrays the seasonal rhythm of light in the Piedmont, which is what matters most for understanding plant growth and planning your gardening activities.

The numbers for Maximum PPFD are estimates based on solar elevation and a representative atmospheric transparency. They will vary with humidity, haze, wildfire smoke, and air quality. If you use a PAR meter at home, you may see slightly lower or higher values depending on those conditions.