Library › Part E · Article 35 in the series · Level 1 · 13 min read

Seeds, germination and propagation

Standfirst: After reading this article you will be able to sow seeds in plugs at the right temperature, light and moisture, recognise and prevent damping-off, and decide when a seedling is ready for its hydroponic system. You will also be able to test a seed packet, calculate how many seeds to sow for the plants you want, and root basil from cuttings.

Before you read: What a plant needs, Choosing a medium.

What a seed is and what it needs

A seed is a resting embryo packed with food and wrapped in a coat. During germination the embryo wakes up and grows on the stored food of the endosperm, the food tissue around it, until it can make its own food by photosynthesis².

Germination is the resumption of growth of that embryo¹. It needs four things: water, oxygen, a suitable temperature, and for some species light, or its absence^1,2. The dry seed first swells as it absorbs water, a step called imbibition². Oxygen matters too: a seed coat that limits the oxygen supply to the embryo is enough to stop germination¹, so a seed sitting in a waterlogged plug can fail for the same reason (this article’s inference). The first visible sign of germination is usually the radicle, the embryonic root, breaking through the coat¹.

A viable seed is a living seed. A viable seed that will not germinate even when water, oxygen and temperature are all right is dormant¹.

Light is the switch for some seeds. Lettuce seed is the classic case: red light promotes its germination and far-red light reverses the effect¹. After alternating red and far-red flashes, nearly all seeds germinated when the last flash was red, and few when it was far-red; the pigment that reads this signal is phytochrome¹. In practice, sow lettuce on the surface or only lightly covered (this article’s inference from¹). The need is not absolute: Harrington’s lettuce emerged from 1.3 cm deep in soil³, and light matters most when the seed is warm, because red light raises the temperature up to which lettuce germinates⁵.

Temperature: the range for each crop

Every species has a minimum temperature below which almost no seed germinates, an optimum at which it germinates fastest, and a maximum. The table gives J. F. Harrington’s data from the University of California, Davis, as reproduced by Oregon State University Extension³, converted from °F to °C (our conversion, rounded), beside the practical ranges of the FAO aquaponics manual⁴.

Harrington’s days to emergence are for seed sown 1.3 cm (half an inch) deep in soil³. Kale is not in the table; it is a cole crop like cabbage⁶, and the cabbage row is the closest guide (this article’s inference).

Lettuce needs a special warning. Harrington recorded lettuce emerging in 2 days at 30 °C but little or no germination at 35 °C³. Resh puts the lettuce optimum at 15 °C to 20 °C and warns that temperatures above 23 °C may induce dormancy². A 2024 review separates thermoinhibition, in which warm seed fails to germinate but sprouts quickly once conditions return to normal, from thermodormancy, a secondary dormancy that sets in after longer exposure to heat; it notes that lettuce genotypes differ in how they cope with heat⁵. The spread between 23 °C and 35 °C reflects cultivar, the conditions the seed matured in, and the test conditions, including light^2,3,5. If lettuce fails to come up in a warm room, try 16 °C until the seedlings emerge, as Resh advises², or chill the seeds on moist paper towels at 2 °C to 5 °C for 48 h before sowing².

Seed forms, storage and a germination test

Seed is sold raw or in prepared forms, three of which matter here. Pelleted seed is coated in clay, which holds moisture around the seed and gives more uniform germination². Primed seed has been hydrated far enough to begin germination and then dried back; priming raises the maximum temperature at which lettuce germinates, but it can shorten the seed’s shelf life⁵. Treated seed has been dressed with a fungicide, and UConn Extension recommends it against damping-off where available¹⁰; seed sold for microgreens is not treated².

Seeds age fastest when warm and moist. A Delaware extension rule says each 1% increase in seed moisture halves the seed’s storage life, and so does each 9 °F (5 °C) rise in temperature⁶. The same source gives a rule of thumb: storage temperature in °F plus relative humidity in percent should be less than 100, for example 50 °F (10 °C) with 50% RH⁶. Keep packets sealed until you sow⁶. Spinach seed stores poorly beyond 1 year; cucumber, kale and tomato seed keep about 3 years⁶. Resh keeps lettuce seed in a refrigerator and warns that it can lose viability within 6 months², while Oregon State says most well-stored seed stays viable for at least a year past its packing season⁷.

To check an older or opened packet, Oregon State’s test uses 10 seeds spread on a damp paper towel, rolled up, sealed in a plastic bag, kept warm and checked after 5 to 10 days⁷. Germination percentage is the number of seeds that sprouted divided by the number tested, times 100. With 10 seeds, each seed is worth 10 percentage points, so a larger sample gives a finer answer (our arithmetic).

Media, containers and sowing

Most hydroponic seedlings start in a small block that goes into the system with the plant, with little transplant setback². The block must hold water and still drain: Resh warns that cubes standing on plastic sheeting sit in collected solution with less oxygen². Choosing a medium compares the materials.

• Stone wool (rockwool) cubes and plugs. Cubes of 2.5 cm × 2.5 cm × 4 cm, 200 to a tray, suit lettuce and herbs; 4 cm cubes, 98 to a tray, suit tomato, pepper and cucumber². Stone wool is slightly alkaline when new². Resh soaks tomato cubes a day ahead in raw water of pH 5.5 to 6.0 and EC below 0.5 mS/cm²; Cornell moistens lettuce plugs with nutrient solution of low pH, such as 4.5, to remove pockets of high pH⁸.
• Oasis Horticubes. Sterile cubes that drain well and have a balanced pH².
• Peat pellets. Compressed peat discs in a nylon mesh that swell to about 4 cm tall after 5 to 10 minutes of soaking, and carry enough nutrients for 3 to 4 weeks².

Every block must be saturated before sowing, because dry spots restrict root growth². Sow one seed per cube for lettuce and fruiting crops^2,8. Basil goes three to four seeds per cube, thinned later to two, and small-seeded herbs such as thyme and oregano six to ten². Do not sow too deep¹⁰. Seeds in a pre-formed hole that is kept moist do not need covering².

High humidity stops the seed drying out before its root is in the medium. Cornell covers lettuce trays with humidity domes for the first 2 days and removes them on day 2, when the radicle has entered the plug; domes are required for raw, unpelleted seed⁸. Bottom heat of 21 °C to 24 °C (70 °F to 75 °F) speeds emergence¹⁰.

Light from the moment of emergence

A seedling that emerges in the dark grows tall and pale. This is etiolation: the stem elongates, the leaves stay small and do not green¹. A lamp switched on a few days late therefore gives thin, stretched seedlings, and Resh notes that spindly seedlings fall over and catch disease easily².

Cornell’s lettuce protocol gives light from sowing: at least 50 µmol m⁻² s⁻¹ for the first 24 hours, which prevented stretching without drying the plug surface, then 250 µmol m⁻² s⁻¹ with a 24-hour photoperiod for the remaining 10 days⁸. That is a daily light integral (DLI), the total light a plant receives in a day, of about 22 mol m⁻² d⁻¹; Cornell accepts shorter photoperiods if the intensity is raised to give the same total⁸. Photosynthesis and respiration explains the DLI arithmetic, and Choosing grow lights covers lamps.

First feeding

A seedling first lives on its seed’s reserves². Once it makes its own food, it needs mineral nutrients, but its roots are tiny and tender. A solution that is too concentrated draws water out of the roots by osmosis (see EC and concentration), and over-fertilised seedlings die of salt injury with shrivelled roots¹⁰.

Sources differ on how dilute to start. Cornell adds fertiliser 24 hours after sowing at an EC of 1.2 mS/cm above the source water, pH 5.8⁸. Resh raises tomato seedlings to EC 1.0 to 1.5 mS/cm as they emerge², and gives lettuce EC 1.5 mS/cm at pH 5.4 to 5.8 once the first true leaves appear². Most of these start with plain water or a dilute solution and raise the EC once the cotyledons, the seed leaves, have opened; Cornell’s feed at 24 hours is the exception⁸.

Damping-off

Damping-off kills seeds and young seedlings. Before emergence, seedlings simply fail to come up; after emergence, they wilt and collapse with a water-soaked or dark lesion at the stem base^10,11. The main causes are Pythium and Rhizoctonia^10,11.

The pathogens thrive where the seedling grows slowly and the medium is wet. Overwatering, a medium that stays wet too long, dripping condensation, overcrowded trays that trap humidity, and a medium colder than 20 °C before germination all promote it¹⁰. Low temperatures and very wet soils delay the growth of the plants and favour infection¹¹. The pathogens travel in splashing water, on tools and on hands¹⁰.

Use clean seed and sterile media, disinfect trays and tools, and do not reuse plug trays that held diseased plants¹⁰. Resh cleans benches with 10% bleach and soaks reused trays in it for at least 30 minutes². Let plugs drain, do not over-sow, keep air moving, and use bottom heat and enough light so seedlings grow quickly through the vulnerable stage¹⁰. Biological fungicides can be added to the medium or drenched at sowing¹⁰. Once symptoms appear, discard the whole tray¹⁰.

Transplanting and hardening

A seedling is ready when true leaves have formed and roots show on the outside of the cube². For lettuce, the range is wide: Cornell transplants on day 11, when leaves overlap and roots have grown through the bottom of the tray⁸; Resh transplants at three to four true leaves, 2 to 3 weeks after sowing²; Kratky sets 1- to 2-week-old seedlings in 5 cm net pots⁹.

In a net pot, the plug must touch the solution. In Kratky’s tank system the lower 1 cm to 2 cm of the net pot sits in the nutrient solution, and the medium is wetted by capillary action until the roots reach the solution⁹. See Kratky and passive systems and Deep water culture.

Hardening, or hardening off, means accustoming plants to outdoor conditions gradually, over 7 to 14 days, mainly by watering less or lowering the temperature¹². It thickens the leaf cuticle and builds up carbohydrate reserves for new root growth¹². A seedling moving from a propagation room to a system in the same room needs no hardening (this article’s inference).

Rooting cuttings

A stem cutting is a piece of a plant that grows its own roots. Because it is part of the same plant, its offspring are uniform, which keeps the cultivar’s traits (a conference paper, read from the abstract only)¹³. Roots form at the cut end in response to the hormone auxin; commercial rooting powders are a synthetic auxin mixed with talc¹.

In a study of sweet basil, cuttings rooted at about 92% in peat moss and 89% in silt, tip and single-node leafy cuttings rooted best, and side shoots rooted more easily than main shoots (abstract only)¹³. Resh suggests cuttings for slow-germinating herbs such as mint and rosemary, which root within several weeks under mist with bottom heat². The abstract does not say whether a rooting hormone was used, so it cannot tell you whether basil needs one. In our MUNCH trial, basil cuttings from adult plants that had not flowered stayed in stone-wool cubes until roots showed through the cubes, and only then went into the systems, both the deep-water containers and the sand bed; nothing was sown directly in either¹⁴.

Worked example: seeds for 24 lettuce positions

You want 24 lettuce plants for a raft. Your seed is a year old, so you test it: 34 of 40 seeds sprout on the paper towel, which is 34 ÷ 40 × 100 = 85% (our arithmetic, method from⁷). You also expect to lose 10% of the seedlings to weak growth or damping-off (an assumption; Resh adds 5% for weak seedlings²).

Seeds to sow = plants wanted ÷ (germination fraction × survival fraction) = 24 ÷ (0.85 × 0.90) = 31.4, rounded up to 32 (our arithmetic). At one seed per cube^2,8, that is 32 cubes. Check: 32 × 0.85 × 0.90 = 24.5 plants (our arithmetic). That is the average outcome: with 32 seeds the chance of at least 24 plants is about two in three, and 35 seeds raise it to about nine in ten (our arithmetic).

Each lettuce cube is 2.5 cm × 2.5 cm², so 32 cubes cover 32 × 6.25 cm² = 200 cm², a block 4 cubes by 8 cubes, or about 10 cm × 20 cm (our arithmetic).

For light, Cornell’s 250 µmol m⁻² s⁻¹ over 24 hours⁸ gives 250 × 3,600 × 24 ÷ 1,000,000 = 21.6 mol m⁻² d⁻¹. A timer set to 16 hours needs 21.6 × 1,000,000 ÷ (16 × 3,600) = 375 µmol m⁻² s⁻¹ to give the same total (our arithmetic, inputs from⁸).

Key numbers

Common mistakes

1. Germinating lettuce in a warm room. Above about 23 °C lettuce seed may go dormant instead of sprouting, and cultivars differ in how much heat they tolerate^2,5.
2. Sowing into dry or partly dry cubes. Dry spots in stone wool restrict root growth².
3. Leaving seedlings in the dark until they are all up. Seedlings that emerge without light stretch and stay pale¹.
4. Keeping humidity domes on and trays wet for days. Wet, crowded, cool trays are where damping-off starts^10,11.
5. Feeding full-strength solution to emerging seedlings. Concentrated solution damages tender roots through salt injury¹⁰.
6. Sowing exactly one seed per plant wanted. Germination is never 100%, so the shortfall shows up as empty positions².

Summary

A seed is an embryo with a food store and a coat; it germinates with water, oxygen, a suitable temperature and, for some seeds such as lettuce, light. Lettuce is the crop most likely to fail from heat. Store seed cool, dry and sealed, and test old packets so you can sow enough to cover losses. Sow into saturated, well-drained plugs, keep humidity high only until the root is in, and give light from emergence. Feed with plain water or a dilute solution first. Prevent damping-off by keeping things clean, drained, uncrowded and growing fast. Transplant when roots show at the plug base, with the plug touching the solution, and use cuttings when you want an exact copy of a plant.

Next in the series: How long things take: growth timelines for ten crops.

References

1. Taiz, L. and Zeiger, E. (2002). Plant Physiology, 3rd ed. Sinauer Associates, Sunderland, MA. Chapters 17, 19 and 23.
2. 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.
3. Harrington, J. F. Soil temperature conditions for vegetable seed germination. Department of Vegetable Crops, University of California, Davis. Reproduced by Oregon State University Extension Service (published April 2013, reviewed 2024). https://extension.oregonstate.edu/catalog/soil-temperature-conditions-vegetable-seed-germination
4. Somerville, C., Cohen, M., Pantanella, E., Stankus, A. and Lovatelli, A. (2014). Small-scale aquaponic food production: Integrated fish and plant farming. FAO Fisheries and Aquaculture Technical Paper No. 589. FAO, Rome. https://www.fao.org/3/i4021e/i4021e.pdf
5. Wei, J., Zhang, Q., Zhang, Y., Yang, L., Zeng, Z., Zhou, Y. and Chen, B. (2024). Advance in the thermoinhibition of lettuce (*Lactuca sativa* L.) seed germination. Plants, 13(15), 2051. https://doi.org/10.3390/plants13152051
6. Johnson, G. (2011). Storing vegetable seed. Weekly Crop Update, University of Delaware Cooperative Extension, 6 May 2011. https://sites.udel.edu/weeklycropupdate/?p=2967
7. Oregon State University Extension Service, Food Hero (2021). Will your seeds grow plants? https://extension.oregonstate.edu/node/236201
8. Brechner, M. and Both, A. J. (2013). Hydroponic Lettuce Handbook. Cornell University Controlled Environment Agriculture Program. https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/8824/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf
9. 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
10. Mercure, P. S. (1998), revised by Pundt, L. (2024). Damping-off of ornamental and vegetable seedlings. UConn Extension Integrated Pest Management Program. https://ipm-cahnr.media.uconn.edu/wp-content/uploads/sites/3216/2024/07/dampingoff.pdf
11. Malathrakis, N. E. and Goumas, D. E. (1999). Fungal and bacterial diseases. In: Albajes, R., Gullino, M. L., van Lenteren, J. C. and Elad, Y. (eds.), Integrated Pest and Disease Management in Greenhouse Crops, pp. 34–47. Kluwer Academic Publishers, Dordrecht. https://doi.org/10.1007/0-306-47585-5_3
12. Lang, K. and Schnabel, C. (2021), from an earlier text by G. Njue. Harden your transplants prior to planting your garden. SDSU Extension, South Dakota State University. https://extension.sdstate.edu/harden-your-transplants-prior-planting-your-garden
13. El-Keltawi, N. E. M. and Abdel-Rahman, S. S. A. (2006). In vivo propagation of certain sweet basil cultivars. Acta Horticulturae, 723, 321–332. https://doi.org/10.17660/ActaHortic.2006.723.45 (abstract only)
14. Sánchez, H. (2024). Method of Up-cycled Non-Circulating Hydroponics (MUNCH): A proof-of-concept experiment with passive organic hydroponics using JADAM liquid fertilizer. PonicLabs technical report (preprint). https://doi.org/10.13140/RG.2.2.26735.65444

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.