Understanding DLI Across the United States: Why Location Matters for Light Management

Light is one of the most critical inputs for plant growth, and Daily Light Integral (DLI) — the total number of photosynthetically active photons plants receive per day — is the best way to quantify it. But DLI is not the same everywhere.
In fact, depending on where you live in the U.S., what season it is, and what your local weather looks like, DLI can vary from just a few mol·m⁻²·day⁻¹ to over 60.

If you’re serious about optimizing plant performance, understanding regional DLI differences is essential.

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What DLI Means in Practice

DLI (measured in mol·m⁻²·day⁻¹) tells you how much usable light plants receive in a 24-hour period. It’s a key metric for planning:

• When and where to plant
• How much supplemental lighting you need
• Whether your plants are receiving enough light for flowering or fruiting

While DLI is influenced by day length, weather, and latitude, the biggest factor is geography — where you are on the map.

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Regional Patterns of DLI in the United States

Here’s how DLI typically varies across major regions of the U.S.:

Region	Summer DLI	Winter DLI	Notes
Southwest (e.g., Arizona, inland California)	55–65 mol·m⁻²·day⁻¹	20–30 mol·m⁻²·day⁻¹	Among the highest sunlight levels year-round. Very little cloud cover.
Central Plains (e.g., Kansas, Oklahoma)	30–50 mol·m⁻²·day⁻¹	10–25 mol·m⁻²·day⁻¹	Plenty of sun in summer, but sharp winter drop.
Northeast & Great Lakes (e.g., New York, Michigan)	30–45 mol·m⁻²·day⁻¹	5–15 mol·m⁻²·day⁻¹	Shorter days and frequent clouds reduce DLI.
Pacific Northwest (e.g., Washington, Oregon)	25–40 mol·m⁻²·day⁻¹	3–10 mol·m⁻²·day⁻¹	Cloud cover and rain significantly reduce light availability.
Mountain West (e.g., Colorado, Utah)	40–55 mol·m⁻²·day⁻¹	15–30 mol·m⁻²·day⁻¹	High elevation boosts DLI, but winter still limits it.

Key insight: Even within the same country, the difference between Arizona and Washington State can be 5–10× in winter sunlight availability.

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Seasonal Effects: Summer vs. Winter

Day length and solar angle change with the seasons, causing major shifts in DLI:

• Summer: Long days and high solar angles boost DLI everywhere.
• Winter: Short days, low sun angles, and frequent clouds can drop DLI below the threshold for many crops.

For example, tomatoes typically need a DLI of 20–30 mol·m⁻²·day⁻¹. That’s easy to achieve outdoors in Arizona year-round — but almost impossible in Michigan without supplemental lighting in winter.

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Why This Matters for Growers

Understanding your local DLI helps you make smarter growing decisions:

• Outdoor growers: Plan planting times and choose crops suited to your climate’s light levels.
• Greenhouse growers: Estimate how much light is lost through glass or plastic (often 30–50%) and plan supplemental lighting.
• Indoor growers: Use DLI targets to design lighting schedules and intensity to mimic natural conditions.

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Measure and Adapt with Real Data

Maps and averages are useful, but they can’t tell you what’s happening at your exact location. Microclimates, shading, or local weather patterns can shift actual DLI significantly.

That’s why tools like the AH-PARDLI PAR & DLI Logger are so valuable. They continuously record light intensity throughout the day and calculate daily DLI — giving you accurate, site-specific data you can use to:

• Adjust lighting schedules
• Optimize planting zones
• Compare seasonal changes year over year

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Bottom line: DLI is one of the most powerful metrics in horticulture, but it’s not a fixed number — it’s shaped by your geography, climate, and season. By understanding regional patterns and measuring actual conditions, you can make data-driven decisions that improve growth, yield, and plant health.