Library › Part E · Article 37 in the series · Level 1 · 11 min read

Growing leafy greens and herbs: lettuce, basil, spinach and others

Standfirst: After reading this article you will be able to run lettuce, basil and spinach from transplant to harvest in any common hydroponic system, with a sourced range for solution strength, pH, temperature, light and spacing. You will know why lettuce leaves burn at the edges, why crops run to seed, why basil blackens in the cold, and what one square metre of lettuce can yield in a year.

Before you read: How long things take, EC and concentration, pH, Light: PPFD, DLI, spectrum and photoperiod, Temperature, humidity and VPD.

Why leaves are the easy crop

Leafy greens and herbs are harvested while the plant is still vegetative, that is, still making leaves rather than flowers. Nothing has to be pollinated and no fruit has to fill, so the crop is short. In the Cornell University raft system, a butterhead lettuce reaches a 150 g head 35 days after seeding³.

The same fact sets the two main risks. Anything that pushes the plant to flower early ruins the crop. And because the leaves grow fast, any limit on what reaches the newest leaves shows at once on the part you sell.

Lettuce and most salad greens are cool-season crops². Basil comes from tropical and subtropical regions¹¹. That one difference explains most of what follows.

Lettuce

Types

Resh describes four basic types: European or Bibb (butterhead), looseleaf, head or iceberg (crisphead), and cos or romaine¹. He calls looseleaf varieties generally the easiest to grow, at 45 to 50 days from seeding to maturity; Bibb takes 45 to 60 days, depending on sunlight¹. Head lettuce takes 80 to 85 days from seed and needs wider spacing, 25 cm × 25 cm¹. A shorter crop makes looseleaf or butterhead the easier first choice (this article’s inference).

Solution, climate and light

Cornell’s raft (pond) system holds the solution at an EC of 1.15 to 1.25 mS/cm above the source water, and a pH of 5.6 to 6.0³. Those trials used reverse-osmosis water; Both reports that the Cornell demonstration greenhouse, on municipal water, ran at 1.65 mS/cm⁴. Kratky’s non-circulating tanks start at 1.5 mS/cm and receive no further feeding⁶. The spread from about 1.2 to 1.65 mS/cm reflects the water source and the system.

Cornell sets the air at 24 °C by day and 19 °C by night, and keeps the solution no warmer than 25 °C³. Older guides give cooler ranges: 8 to 24 °C air², or 12 to 20 °C⁵. Resh splits by type: Bibb wants 18 °C at night and 21 to 24 °C on sunny days, while looseleaf types tolerate up to 27 °C¹. A cool root zone buys room for warm air: Thompson and colleagues, summarised by Both, grew quality lettuce in air above 24 °C only when the solution stayed below 20 °C⁴.

Cornell’s light target is a daily light integral (DLI, the total photosynthetic light per square metre per day) of 17 mol m⁻² d⁻¹³. More caused damage, as the next section explains⁴.

Spacing and timing

Cornell transplants 11-day-old seedlings into the pond at 97 plants/m², then respaces them to 38 plants/m² on day 21 for the last two weeks³. Both’s research trials used about 80 and then 35 plants/m²⁴. Fixed spacings are common in smaller systems: Resh gives 15 cm × 20 cm for Bibb lettuce¹, and Kratky cuts holes at 20 cm × 30 cm in his tank covers⁶. In plants per square metre, those are about 33 and 17 (our conversion). Kratky’s leaf and semi-head lettuce is ready about five weeks after transplanting⁶.

Tipburn: a calcium delivery problem

Tipburn is the browning and death of the margins of young, inner lettuce leaves. Calcium moves almost only in the xylem, carried by the transpiration stream; transport in the phloem is negligible⁷. Organs that transpire little, such as young inner leaves, therefore go short first, and tipburn is a typical result⁷; see Water in the plant.

Both puts the rule in numbers: lettuce must transpire at least 400 mL of water per gram of dry mass it accumulates to stay free of tipburn⁴. A 150 g Cornell head holds nearly 7 g of dry matter³, so, by our estimate, it must move roughly 2.8 L of water or more through its leaves over its life.

Light drives growth faster than still air drives transpiration. In the Cornell trials, tipburn appeared when the plants received more than 17 mol m⁻² d⁻¹⁴. Without fans, the Cornell team could not go above 12 mol m⁻² d⁻¹³. Paddle fans blowing air straight down onto the crop raise transpiration and carry more calcium to the young leaves³. Cornell warns that these figures are for one cultivar, ‘Ostinata’, no longer sold, and that the safe DLI depends on cultivar, spacing and airflow³.

The solution matters too. Jones advises more calcium, a lower potassium-to-calcium ratio and a low EC². High concentrations of competing cations (ammonium, potassium, magnesium) or of salts can induce calcium deficiency⁷. High humidity also reduces transpiration². Diagnosis is covered in Diagnosing nutrient disorders.

Bolting: when the plant switches to flowering

Bolting is the fast elongation of the stem that will carry the flowers. Some plants keep a compact rosette in short days and elongate and flower only in long days; the plant hormone gibberellin regulates that elongation⁸. Spinach is the textbook case: moved into long days, its shoots begin to elongate after about 14 long days⁸. A spring crop meets lengthening days, so it bolts (this article’s inference). Cornell spinach work in NFT held a 16-hour photoperiod (06:00 to 22:00) to prevent early bolting (an abstract reproduced in³); the abstract does not explain why 16 hours was short enough for its cultivar.

Baby-leaf spinach shows the other side: Cornell’s spinach project recommended a photoperiod long enough to trigger bolting, which raised productivity in a 14- to 16-day crop⁹. Crops cut more than once need bolt-resistant cultivars⁹; the project compared ‘Alrite’, fast early but prone to bolt, with ‘Eagle’, slower and tolerant of long photoperiods⁹.

For lettuce, the sources used here point mainly to heat: Jones reports bolting above 25 °C air², and Mason flower stalks above 27 °C⁵. Resh places the head-lettuce bolting threshold at 25 to 28 °C, and lower when a cover reduces the light¹. On a Caribbean farm, chilling the solution to about 18 °C under 32 to 35 °C air delayed bolting by 4 to 5 days¹. Cultivar choice is the cheaper lever: Resh names ‘Rex’¹, and Jones lists slow-bolting cultivars such as ‘Buttercrunch’².

Basil

Resh lists sweet Italian, Thai, cinnamon, lemon and purple basils among the herbs suited to hydroponic greenhouses¹; sweet basil (Ocimum basilicum) includes the Genovese types¹². Resh’s herb guidance is pH 5.8 to 6.2, EC 1.6 to 2.5 mS/cm and 18 to 24 °C¹. Mason gives pH 5.5 to 6.5 and ideally 20 to 24 °C, with plants 20 cm apart⁵; Resh spaces smaller herbs at 15 cm × 15 cm in NFT¹.

EC matters less than these ranges suggest. In an Iowa State trial summarised by its researcher in a trade magazine, sweet, holy and lemon basil grew in NFT at 0.5, 1, 2, 3 or 4 mS/cm, and growth showed no significant differences¹⁰. The same summary reports that basil “responds very favorably to high light” (~15 versus ~7 mol m⁻² d⁻¹)¹⁰.

Basil is chilling-sensitive: temperatures below 12 °C cause strong wilting, discoloured leaves and loss of aroma (Begum and colleagues, citing Fratianni and colleagues, 2017)¹¹. Mason’s advice to store cut basil at about 5 °C⁵ conflicts with that; this article follows the more recent chilling evidence.

The main disease is downy mildew, caused by Peronospora belbahrii, which is seed-borne and spreads on air-blown spores¹². It needs at least 4 hours of leaf wetness to infect and more than 7.5 hours above 95% humidity at night to sporulate¹². Sweet basil is the most susceptible type, and bred Genovese varieties now carry intermediate to high resistance¹². Control is in Pests and diseases.

Harvest before flowering starts. For the first harvest, remove the shoot tips with one or two nodes; side shoots then develop for a second harvest about a month later⁵. In our own 2019 trial, basil seedlings in a 12 L deep-water culture tank fed with dissolved vermicompost (starting EC 0.6 mS/cm) grew into fully grown plants over 50 days, despite aphids and leaf discoloration¹³. It was one small trial without a control.

Spinach

Spinach prefers cool, shaded conditions, best between 15 and 19 °C, at pH 6.0 to 7.0, with plants 8 to 10 cm apart, according to Mason⁵. Cornell’s baby-leaf work points elsewhere: a 14- to 16-day crop at a DLI of 17 mol m⁻² d⁻¹, 1,500 plants/m², and root-zone and air temperatures of 20 to 25 °C depending on cultivar⁹. In one of its experiments, the pond solution was set near 1.2 mS/cm⁹. In this article’s reading, the two describe different products: full-size plants versus a dense stand cut young.

Root rot is one of the two reasons hydroponic spinach is rare. Cornell found that uneven germination and early root rot by Pythium aphanidermatum had kept hydroponic spinach out of commercial production in the United States⁹. Pythium is a water mould whose swimming spores (zoospores) find the roots and are released again into the solution, spreading the infection⁹. In one Cornell test, healthy 12-day-old seedlings placed in a pond that had just held an infected crop had all collapsed after 5 days⁹. P. aphanidermatum is most pathogenic to hydroponic spinach at 27 °C, while P. dissotocum peaks at 17 to 22 °C¹⁵. In Cornell’s tests, infected and healthy plants did not differ at 18 or 24 °C solution, but did at 30 °C⁹. Their large-leaf protocol: grow seedlings cleanly for 9 days, then transplant into a pond cooled to 18 °C and treated to keep zoospores low⁹.

Kale, rocket, chard, mint and coriander

Resh describes chard and arugula (rocket) grown in NFT alongside lettuce and basil at a rooftop greenhouse¹. In one salad-green bed, arugula and spinach were sown weekly, with 3 weeks per crop and one harvest per crop¹. Kale grows in conditions similar to cabbage, is likely to do best in aggregate media, and can reach 50 cm, needing support⁵. Mint grows well at 15 to 20 °C and pH 6.5 to 7.0, and NFT has been very successful⁵. Resh notes that conditions similar to those for lettuce are generally good for herbs¹. Coriander grew together with basil in our 2016 urine-fed (anthroponic) deep-water culture trial, with adequate aeration and pH control as the conditions for success¹⁴.

Nitrate in leaves

Leaves can store nitrate faster than they convert it into protein. The conversion needs the enzyme nitrate reductase and energy carriers from photosynthesis, so high light favours it and low light leads to accumulation¹⁶. The European Union caps nitrate in a few leafy vegetables. Under Regulation (EU) 2023/915, which keeps the limits first set by Regulation 1258/2011 (tabulated by Luetic and colleagues), the limits for fresh lettuce other than iceberg types are 5,000 mg/kg when grown under cover and harvested from October to March, and 4,000 mg/kg from April to September^17,20. Fresh spinach is capped at 3,500 mg/kg and rocket at 7,000 or 6,000 mg/kg by season^17,20.

The two levers are light and the share of ammonium in the nitrogen supply. In one study, raising ammonium from 0.1 to 0.5 of the nitrogen cut shoot nitrate by 87%, but shoot biomass fell by 50%¹⁸. Both solutions in that study also carried extra sodium and chloride, as in a urine-recycling scenario¹⁸. High ammonium can also induce calcium deficiency⁷, which matters for tipburn. See Nitrogen form: nitrate and ammonium.

Harvest and after

Lettuce is harvested as a whole plant; Cornell packs it in clamshells with or without roots and stores it at 40 °F (about 4 °C)³. Mason recommends rapid cooling and storage at 2 to 4 °C under high humidity⁵. Spinach can be cut and regrown: Cornell found significant benefits from a second cutting⁹. Basil is cut in the morning while it is cool, and kept cool afterwards, though not below its chilling threshold^5,11.

Soilless does not mean pathogen-free. Aquaponic lettuce carried fewer spoilage and faecal microorganisms than soil-grown lettuce, but the authors stress good agricultural practice whatever the growing method¹⁹. Cornell cleans its floats between crops with a weak (2%) bleach solution³.

Worked example: one square metre of raft lettuce

Density. Take Cornell’s final spacing of 38 plants/m²³. A home raft is rarely respaced, so assume the plants stay at 38/m² from transplant to harvest.

Light. Target a DLI of 17 mol m⁻² d⁻¹³ over a 16-hour photoperiod (our assumption; Cornell reaches the DLI with sunlight plus supplemental light³). PPFD = 17 × 10⁶ ÷ (16 × 3,600) = 295 µmol m⁻² s⁻¹ (our calculation). Without a fan blowing down on the crop, cap the DLI at 12³, which is 208 µmol m⁻² s⁻¹ over 16 hours.

Solution. EC 1.15 to 1.25 mS/cm above your source water and pH 5.6 to 6.0³; air 24/19 °C and solution at or below 25 °C³.

Crop time and yield. Cornell plants spend 24 days in the pond and are harvested at about 150 g³. With harvest and replanting on the same day, 1 m² carries 365 ÷ 24 = 15.2 crops a year (our calculation). That gives 15.2 × 38 = 578 heads, or 578 × 0.150 = 87 kg of lettuce per square metre per year (our estimate).

Check. With Cornell’s respacing (10 days at 97/m², then 14 days at 38/m²), each head occupies 10 ÷ 97 + 14 ÷ 38 = 0.47 m²·days, so 1 m² gives 774 heads or 116 kg a year (our estimate). Cornell’s own facility sizing, 1,245 heads per day from 660 m²³, works out to 689 heads or 103 kg per square metre per year (our calculation). The respaced figures assume no empty space; real facilities lose area to aisles and turnover.

These yields need seedlings raised elsewhere for 11 days³, and they assume the cultivar tolerates 17 mol m⁻² d⁻¹ without tipburn.

Key numbers

Common mistakes

1. Pushing light without moving air. Fast growth in still air outpaces transpiration, so calcium does not reach the inner leaves and tipburn follows.
2. Letting the solution warm up for lettuce and spinach. Warm roots raise the risk of bolting in lettuce and favour Pythium aphanidermatum in spinach^1,9,15.
3. Putting basil in the refrigerator. Basil is chilling-sensitive, and temperatures below 12 °C discolour the leaves and dull the aroma¹¹.
4. Raising EC to make basil grow faster. Basil grew equally well from 0.5 to 4 mS/cm, while it responded strongly to light¹⁰.
5. Growing winter greens under weak light on a nitrate-only feed. Without enough light, nitrate reductase cannot keep up, and nitrate builds up in the leaves you eat.

Summary

Leafy greens and herbs are harvested before flowering, which makes them short crops. Lettuce grows well at about 1.2 to 1.65 mS/cm and pH 5.6 to 6.0, with a cool root zone and a DLI near 17 mol m⁻² d⁻¹^3,4. That light is safe only with air moving down over the crop, because tipburn is a failure of calcium delivery by transpiration. Spinach bolts in long days; lettuce bolts mainly in heat. Basil tolerates a wide EC range, wants warmth and light, and is damaged below 12 °C^10,11. Spinach needs a cool, clean root zone because Pythium thrives in warm solution. More light lowers nitrate in leaves. By our estimate, one square metre of raft lettuce can yield roughly 87 to 116 kg a year.

Next in the series: Growing fruiting crops: tomato, cucumber, pepper.

References

1. Resh, H. M. (2013). Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower, 7th ed. CRC Press, Boca Raton. ISBN 978-1-4398-7869-9.
2. Jones, J. B. Jr. (2005). Hydroponics: A Practical Guide for the Soilless Grower, 2nd ed. CRC Press, Boca Raton.
3. Brechner, M., Both, A. J. and Cornell CEA staff (2013). Hydroponic Lettuce Handbook. Cornell University Controlled Environment Agriculture. https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/8824/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf
4. Both, A. J. (2003). Ten years of hydroponic lettuce research. Rutgers, The State University of New Jersey. https://www.haifa-group.com/sites/default/files/2024-03/AJBothTenYearsofHydroponicLettuceResearch2003.pdf
5. Mason, J. Commercial Hydroponics, 3rd ed. ACS Distance Education. Year not printed in the corpus copy.
6. Kratky, B. A. (2009). Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae, 843, 65–72. https://doi.org/10.17660/ActaHortic.2009.843.6
7. Wiesler, F. (2012). Nutrition and quality. In Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed., chapter 9. Academic Press, London. https://doi.org/10.1016/B978-0-12-384905-2.00009-1
8. Taiz, L. and Zeiger, E. (2002). Plant Physiology, 3rd ed. Sinauer Associates, Sunderland, MA. Chapter 20, Gibberellins.
9. Albright, L. D., de Villiers, D. S., Langhans, R. W. et al. (2005). A commercially viable controlled environment agriculture (CEA) spinach production system. Final report to the New York State Energy Research and Development Authority, Cornell University. https://cea.cals.cornell.edu/files/2019/07/Spinachfinal-report.pdf
10. Walters, K. J. (2016). Managing electrical conductivity (EC) for hydroponic basil production. Greenhouse Grower, 14 January 2016 (trade magazine summary of research later published as Walters and Currey 2018, HortScience 53(9), 1319–1325, https://doi.org/10.21273/HORTSCI13126-18 https://www.greenhousegrower.com/?p=126303
11. Begum, F. U., Skinner, G., Smieszek, S. P. et al. (2023). Improved chilling tolerance in glasshouse-grown potted sweet basil by end-of-production, short-duration supplementary far red light. Frontiers in Plant Science, 14, 1239010. https://doi.org/10.3389/fpls.2023.1239010
12. Pundt, L. and Allen, J. (2013, revised 2024). Downy mildew on basil in the greenhouse. UConn Extension Integrated Pest Management Program fact sheet. https://ipm-cahnr.media.uconn.edu/wp-content/uploads/sites/3216/2024/06/basildownymildewghs.pdf
13. Sánchez, H. (2019). Cultivating *Ocimum basilicum* in a hydroponics deep-water culture (DWC) system using dried vermicompost powder as the nutrient source. PonicLabs technical report. https://doi.org/10.13140/RG.2.2.31871.66725
14. Sánchez, H. (2016). *Ocimum basilicum* and *Coriandrum sativum* cultivation in a decoupled anthroponics system. PonicLabs technical report. https://www.researchgate.net/publication/312083724
15. Albajes, R., Gullino, M. L., van Lenteren, J. C. and Elad, Y. (eds) (2000). Integrated Pest and Disease Management in Greenhouse Crops. Developments in Plant Pathology 14. Kluwer Academic Publishers, Dordrecht. Chapter "Managing the greenhouse, crop and crop environment", printed p.111, citing Bates and Stanghellini (1984). https://doi.org/10.1007/0-306-47585-5
16. Akter, N., Cammarisano, L. and Ahamed, M. S. (2026). Interactive effects of electrical conductivity and light intensity on growth, yield, and nutrient dynamics of hydroponic lettuce. Scientific Reports, 16, 14803. https://doi.org/10.1038/s41598-026-44508-2
17. Luetic, S., Knezovic, Z., Jurcic, K. et al. (2023). Leafy vegetable nitrite and nitrate content: potential health effects. Foods, 12(8), 1655. https://doi.org/10.3390/foods12081655
18. Schiefloe, M., Jakobsen, Ø. M., Pannico, A., Quadri, C. and Jost, A.-I. K. (2023). From urine to food and oxygen: effects of high and low NH₄⁺:NO₃⁻ ratio on lettuce cultivated in a gas-tight hydroponic facility. Frontiers in Plant Science, 14, 1229476. https://doi.org/10.3389/fpls.2023.1229476
19. Sirsat, S. A. and Neal, J. A. (2013). Microbial profile of soil-free versus in-soil grown lettuce and intervention methodologies to combat pathogen surrogates and spoilage microorganisms on lettuce. Foods, 2(4), 488–498. https://doi.org/10.3390/foods2040488
20. European Commission (2023). Commission Regulation (EU) 2023/915 of 25 April 2023 on maximum levels for certain contaminants in food and repealing Regulation (EC) No 1881/2006. Official Journal of the European Union, L 119, 5 May 2023, p. 103. https://eur-lex.europa.eu/eli/reg/2023/915/oj

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.