DLI Targets by Crop Chart for Greenhouse Growing

Daily light integral is the total number of photosynthetically usable photons landing on one square meter over a whole day, measured in moles per square meter per day. It is the single most useful number in greenhouse growing, because unlike instantaneous light readings it accounts for both intensity and duration, which is exactly how a plant experiences light.

The conversion from an instantaneous reading is straightforward. PPFD, photosynthetic photon flux density, is measured in micromoles per square meter per second. Multiply PPFD by the number of hours, then by 3,600 seconds per hour, then divide by one million to get from micromoles to moles.

The conversionPPFD x h x 0.0036

PPFD in micromoles per square meter per second, multiplied by the photoperiod in hours, multiplied by 0.0036, gives the daily light integral in moles per square meter per day. A fixture delivering 250 PPFD for 16 hours produces a DLI of 14.4.

Figure 1. DLI target and the PPFD needed to reach it, by crop · 30 rows

Figure 1. DLI target and the PPFD needed to reach it, by crop
Crop	Target DLI	PPFD at 12 h	PPFD at 16 h	PPFD at 18 h	Below this, growth stalls
Tomatoes	22 to 30	509 to 694	382 to 521	340 to 463	18
Peppers	20 to 30	463 to 694	347 to 521	309 to 463	15
Cucumbers	20 to 30	463 to 694	347 to 521	309 to 463	15
Eggplant	20 to 26	463 to 602	347 to 451	309 to 401	15
Melons	22 to 30	509 to 694	382 to 521	340 to 463	18
Summer squash	20 to 26	463 to 602	347 to 451	309 to 401	15
Green beans	18 to 24	417 to 556	313 to 417	278 to 370	14
Peas	14 to 20	324 to 463	243 to 347	216 to 309	10
Strawberries	17 to 22	394 to 509	295 to 382	262 to 340	12
Lettuce	12 to 17	278 to 394	208 to 295	185 to 262	6
Spinach	12 to 17	278 to 394	208 to 295	185 to 262	6
Kale	12 to 17	278 to 394	208 to 295	185 to 262	7
Swiss chard	12 to 16	278 to 370	208 to 278	185 to 247	7
Arugula	10 to 14	231 to 324	174 to 243	154 to 216	6
Bok choy	10 to 14	231 to 324	174 to 243	154 to 216	6
Carrots	12 to 17	278 to 394	208 to 295	185 to 262	8
Beets	12 to 16	278 to 370	208 to 278	185 to 247	8
Radishes	10 to 14	231 to 324	174 to 243	154 to 216	6
Scallions	10 to 14	231 to 324	174 to 243	154 to 216	6
Basil	12 to 20	278 to 463	208 to 347	185 to 309	8
Cilantro	10 to 14	231 to 324	174 to 243	154 to 216	6
Parsley	10 to 14	231 to 324	174 to 243	154 to 216	6
Microgreens	6 to 12	139 to 278	104 to 208	93 to 185	4
Seedlings and transplants	10 to 14	231 to 324	174 to 243	154 to 216	6
Rooting cuttings	4 to 8	93 to 185	69 to 139	62 to 123	2
Geraniums	12 to 20	278 to 463	208 to 347	185 to 309	9
Petunias	10 to 18	231 to 417	174 to 313	154 to 278	8
Marigolds	12 to 18	278 to 417	208 to 313	185 to 278	9
Orchids, phalaenopsis	4 to 6	93 to 139	69 to 104	62 to 93	2
Ferns and shade foliage	3 to 6	69 to 139	52 to 104	46 to 93	2

PPFD figures are the micromoles per square meter per second a fixture must deliver at the canopy, not at the fixture. Light falls off with the square of distance, so a strip 12 inches above a tray delivers roughly a quarter of what it delivers at 6 inches. The final column is the DLI below which the crop survives but effectively stops adding usable growth.

Why a greenhouse in winter is a dark place

The reason this chart matters is a mismatch nobody warns beginners about. Outdoor daily light integral at mid latitudes runs from roughly 45 to 60 moles per square meter per day at midsummer down to 5 to 15 at midwinter. Now apply the greenhouse: glazing transmits 74 to 90 percent, the frame and glazing bars shade another 5 to 12 percent, and dirt on the glazing can take 10 percent more.

Put those together and a midwinter greenhouse can be receiving 4 to 9 moles per square meter per day at bench level. Against a lettuce target of 12 to 17, that is not a marginal shortfall, it is a factor of two or three. This is the real reason winter greenhouse crops sit still for weeks: not cold, which heating solves, but light, which heating does nothing for.

Figure 2. Typical DLI inside a greenhouse through the year, mid latitudes · 7 rows

Figure 2. Typical DLI inside a greenhouse through the year, mid latitudes
Season	Outdoor DLI	Inside, clean twin-wall	Supports without supplement
Midwinter	5 to 15	3 to 10	Dormant hardy greens only, little active growth
Late winter	15 to 25	10 to 17	Salad crops grow slowly, seedlings need help
Early spring	25 to 35	17 to 24	Good salad growth, adequate for transplants
Late spring	35 to 50	24 to 34	Everything, and shade cloth becomes necessary
Midsummer	45 to 60	30 to 41	Everything, with 50 percent shade on fruiting crops
Early autumn	30 to 40	20 to 27	Good growth, last window for a fast autumn crop
Late autumn	15 to 25	10 to 17	Establishment only, growth is slowing hard

Inside figures assume 80 percent glazing transmission and 15 percent structural shading, which is typical for a hobby kit with a heavy frame. Actual figures vary strongly with latitude and cloud cover.

How much light does a fixture actually deliver?

Fixture marketing is close to useless here, because the two numbers usually quoted, wattage and lumens, tell you almost nothing horticulturally. Wattage is energy consumed. Lumens are weighted to human eye sensitivity, which peaks in green, the part of the spectrum plants use least. The only figure that matters is PPFD at the distance the plants will actually be, and the only reliable way to know it is to measure it with a quantum sensor such as the DANOPLUS Quantum PAR Meter, PPFD Tester.

As a working approximation for the fixtures in this category: a single 4 ft LED strip at about 20 W delivers roughly 100 to 150 PPFD at 6 inches directly below it and 40 to 60 at 12 inches, over a band only a few inches wide. That is why seed-starting racks use two or more strips per shelf rather than one, and why kits such as the Barrina 4 ft LED Grow Lights, 5000 K, 6-Pack ship in packs of six. A single strip down the middle of a 1020 tray leaves the outer rows visibly leggier than the center within ten days.

Note

Lux meters are not PAR meters

A lux meter costs a third of a quantum sensor and can be genuinely useful, but only with a conversion factor specific to your light source, and the factor varies by nearly 50 percent between sources. Divide lux by roughly 54 for sunlight, 74 for a white LED or T5 fluorescent, and 82 for high pressure sodium, to get an approximate PPFD. Never compare a lux reading under sunlight against one under a lamp.

Photoperiod is a separate variable from DLI

Two fixtures can deliver the same daily light integral with completely different results, because some crops respond to daylength independently of total light. Lettuce and spinach bolt in response to long days. Onions form bulbs in response to daylength. Many ornamentals initiate flower buds on a daylength trigger.

For food crops under lights the practical rules are simple. Seedlings and transplants do well at 14 to 16 hours. Fruiting crops do well at 16 to 18 hours, and tomatoes will take 18 but show injury symptoms under continuous light. Leafy greens can take 16 to 18 hours to hit their DLI target in winter, and the bolting risk is low in winter because temperature is the stronger trigger. A programmable timer is what makes any of this repeatable.

Meeting a DLI target

Two fixture options at different scales, the measurement tool that tells you whether they are working, and the hangers that let you set the distance correctly.

Bench and rack scale

Barrina 4 ft LED Grow Lights, 5000 K, 6-Pack

$94.99

Six 4 ft strips at 5000 K. Two per shelf, held two to four inches above the canopy, is the arrangement that reaches a seed-starting DLI in the 10 to 14 range.
Wire shelving scale

Barrina T8 2 ft Grow Lights, 5000 K, 6-Pack

$71.24

Six 2 ft strips, the right length for the 24 inch wire shelving most seed-starting racks are built from.
Measurement

DANOPLUS Quantum PAR Meter, PPFD Tester

$79.99

A quantum sensor reading PPFD directly. The only way to know what a fixture delivers at the canopy rather than what its box claims.
Setting the distance

OneChoi Adjustable Rope Light Hangers, 8 ft

$35.00

Ratchet hangers let the fixture start two inches above the seed and rise weekly. Distance matters more than any other adjustable variable.

Convert between PPFD, photoperiod and DLI for your own fixture with the DLI and PPFD calculator, see the fixture picks in best grow lights for seed starting, and compare technologies in LED versus fluorescent. The shade side of the same equation is in the shade cloth percentage chart.

Related on this site

Common questions

6 answers

+ What DLI do tomatoes need?

Tomatoes want 22 to 30 moles per square meter per day and will stall below about 18. That is among the highest requirements of any greenhouse crop, which is why tomatoes are a summer crop under glass and why winter tomato production needs serious supplemental lighting rather than a few strips. Over an 18 hour photoperiod, 22 mol/m2/day requires a sustained 340 micromoles per square meter per second at the canopy.

+ How do I convert PPFD to DLI?

Multiply PPFD in micromoles per square meter per second by the photoperiod in hours, then by 3,600, then divide by one million. The shortcut is to multiply PPFD by hours by 0.0036. A fixture delivering 200 PPFD for 16 hours gives a DLI of 11.5, and one delivering 400 PPFD for 12 hours gives 17.3. Both numbers matter: the same DLI at different intensities can produce different results.

+ What is the minimum DLI for lettuce?

Lettuce grows well at 12 to 17 moles per square meter per day and survives down to about 6, below which it holds but adds almost no usable weight. That threshold is the practical limit on winter salad production in an unlit greenhouse: at mid latitudes an inside DLI of 3 to 10 in deep winter puts you at or below it for several weeks, which is why winter lettuce sits still rather than dying.

+ Is more light always better?

No. Above a crop specific saturation point, extra light produces no extra photosynthesis but does produce extra heat and extra water demand, and eventually photoinhibition, where the photosynthetic apparatus is damaged. That is why summer shade cloth increases yield rather than reducing it: cutting light from 45 to 30 moles per square meter per day costs a tomato crop nothing and saves it substantial water stress.

+ How far above seedlings should a grow light be?

For LED strips of the type used on seed-starting racks, two to four inches above the leaf canopy, raised weekly as the seedlings grow. Light falls off with the square of distance, so moving a fixture from 4 inches to 8 inches cuts delivered PPFD to roughly a quarter. Leggy, stretched seedlings are the visible symptom of a fixture too far away, not of too little running time.

+ Does a greenhouse need supplemental light in winter?

For active growth, usually yes at mid and high latitudes. Inside daily light integral in deep winter commonly falls to 3 to 10 moles per square meter per day, against 12 to 17 for salad crops. Supplemental light over a bench, rather than over the whole house, is the economic answer: lighting 12 square feet of propagation bench costs a small fraction of lighting 180 square feet of greenhouse.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.

How this page was researched

Everything here is researched general information for growing vegetables, herbs, fruit and flowers, not professional horticultural, structural or electrical advice. Figures come from published manufacturer specifications, published horticultural references and verified owner reviews. We do not claim hands-on testing of the equipment described. Temperature, light and heating requirements vary by climate, crop and structure, so treat every number here as a starting point to check against your own conditions. Follow the label on any product you apply to an edible crop, follow local code for electrical and gas work, and consult a qualified professional for anything structural.