Library › Part E · Article 36 in the series · Level 1 · 13 min read

How long things take: growth timelines for eleven crops

Standfirst: After reading this article you will be able to read a published crop timeline and say what it actually measured, find a sourced range from sowing to harvest for eleven common hydroponic crops, and turn a timeline into a weekly sowing plan that gives a steady harvest. You will also know what to record so that your own notes become a better guide for your system than any published table.

Before you read: Seeds, germination and propagation.

A timeline is a range, not a number

A plant grows through chemical reactions, and most chemical reactions run faster when they are warmer. Growers and crop scientists capture this with thermal time, also called the thermal integral: the sum, day by day, of the mean temperature above a threshold called the base temperature¹². It is counted in degree days, where one degree day is 1 °C above the base temperature held for 24 hours¹². The idea only works where the speed of development rises in a straight line with temperature, and only for temperatures above the base¹².

For lettuce, a 1976 field study in Brazil (in Portuguese, cultivar White Boston) estimated a base temperature of 6 °C from emergence to transplant and 10 °C from transplant to harvest¹³. With a base of 10 °C¹³, a day at a mean of 20 °C adds 10 degree days and a day at 15 °C adds only 5. The simple model therefore predicts that the cooler crop needs twice as many days to reach the same stage (our arithmetic from¹² and¹³). The same study also found that the relation was not a straight line: the rate of development peaked at about 22 °C and fell above it¹³. Warmth shortens a crop only up to a point. The article Temperature, humidity and VPD explains how to set those temperatures.

Light sets the other limit. The daily light integral (DLI) is the number of moles of light particles that reach one square metre of crop in one day, explained in Light: PPFD, DLI, spectrum and photoperiod. The Cornell lettuce handbook targets 17 mol m⁻² d⁻¹¹. Without fans blowing air down onto the plants, the Cornell team could not go above 12 mol m⁻² d⁻¹ without tipburn in the cultivar ‘Ostinata’, no longer sold, and the lower light “slows the rate of growth”¹. Day length matters for flowering crops too. In an indoor farm at 21 °C, strawberry ‘Albion’ under a 16-hour day gave its first ripe fruit 17 days earlier than under a 12-hour day, at the highest light level tested⁹.

Two more things move the date. The first is the cultivar: one hydroponics manual says lettuce “will mature in 40 – 85 days depending on variety”⁵. The second is the size you decide to harvest at. In a Cornell kale trial, reported in a trade magazine, plants were harvested as baby leaf two weeks after transplant and at juvenile size after four weeks⁸. The end of a timeline is a choice, so a timeline without a harvest size is incomplete.

The rule that follows: every time in this article is printed with the conditions of the trial that produced it. When your conditions differ, expect your time to differ in the direction the mechanism predicts: longer when cooler (above the optimum, also longer when hotter), longer in lower light, longer for a bigger harvest.

What “days to maturity” on a seed packet means

Seed packets print one number, and it does not always count from the same day. One seed company defines it as the “Average number of days from seeding date to harvest”, and, for a transplanted crop, as the number of days from the transplant date¹¹. If a crop can be either direct-seeded or transplanted, the figure refers to direct seeding¹¹. The same company lists one bell pepper at 66 days for green fruit and 86 days for yellow ripe fruit, counted from transplanting outdoors to the first picking of full-size fruit¹¹.

So a pepper packet figure leaves out the seedling weeks. In one greenhouse schedule, peppers spent 51 days between sowing and planting into their final slabs (our arithmetic from the dates in⁴). This pepper’s figure is also a field figure, counted from transplanting outdoors¹¹, so it carries the season and weather of wherever it was measured. Indoors, temperature and light are different, so the number transfers poorly. In our view it is most useful for ranking cultivars as earlier or later, and least useful as a calendar.

Short crops and long crops

Leafy crops such as head lettuce and spinach are usually harvested once. For them the time to harvest and the length of the crop are the same number. Leaf crops that are picked again and again have a harvest window as well. A practical guide from the Food and Agriculture Organization of the United Nations (FAO) says basil leaf harvest starts when plants reach 15 cm and continues for 30 to 50 days³.

Fruiting crops pass through stages. First the plant builds leaves, stem and roots (vegetative growth). Then it forms flower buds, the flowers open and are pollinated, and fruit sets. The fruit then fills and ripens before the first pick, and an indeterminate plant, one that keeps producing new flower clusters, goes on flowering and fruiting for months³. In a greenhouse schedule from Resh’s handbook, tomatoes sown in mid-November are moved into their final slabs in the first week of January “when flower buds appear”, and the first harvest comes in the third week of February⁴. That is 48 to 61 days from sowing to visible flower buds, about 45 days from there to the first ripe fruit, and 93 to 106 days from sowing to first harvest (our arithmetic from the dates in⁴). The last harvest in that schedule is on 21 November, about nine months after the first⁴.

For a fruiting crop, “time to harvest” and “crop length” are therefore two different numbers. The first is measured in weeks; the second in months.

Timelines for eleven crops from trials, schedules and guides

Each row in the table below comes from one source, with that source’s conditions. The FAO rows come from a practical guide built on growers’ and researchers’ experience, not from one trial³. The Resh rows are a handbook’s recommended greenhouse calendars⁴. Where a source does not state a value, the cell says so; we did not fill gaps with estimates. Where a guide gives a “growth time” without saying whether it counts from sowing or from transplant, the figure sits in the last column with that warning.

All Resh intervals are our arithmetic from the calendar dates in his schedules⁴. The Cornell 24 days is 35 minus 11¹.

Three patterns stand out. Leafy crops and herbs spend 1 to 3 weeks as seedlings and 2 to 5 weeks in the system^1,2,6,7,8. Fruiting crops in Resh’s greenhouse schedules take 61 to 137 days from sowing to first pick, cucumber fastest and pepper slowest, and are then harvested for months (our arithmetic from⁴). The FAO figures are shorter than Resh’s for tomato, perhaps because Resh’s tomatoes are sown in November and raised through winter, when he recommends supplementary lighting for seedlings in northerly latitudes⁴; this is our inference, not a comparison either source makes.

The parsley rows differ by a factor of about three, and they also count from different days (“after transplant” against “from planting”)^3,5. The strawberry row is mostly blank because the paper gives days to first harvest only in a figure, not in its text⁹. Coriander is missing because we found no trial that gave a timeline we could check.

Turning a timeline into a schedule

A timeline tells you how long one plant takes. A schedule tells you when to sow so that something is always ready. The method is the same for any single-harvest crop: sow a fixed batch every week, and the system fills up with one batch at each stage.

Worked example. A household wants four heads of lettuce a week. The inputs come from the table. Seedlings take about two weeks: Kratky transplants seedlings 1 to 2 weeks old², and Cornell moves them on day 11¹. Plants then spend about 3½ to 5 weeks in the system: 24 days at Cornell under 17 mol m⁻² d⁻¹¹ and about five weeks in Kratky’s tanks²; one manual gives four weeks in summer without saying whether it counts from sowing or from transplant⁵. All the results below are this article’s own arithmetic.

1. Sowing rate. Sow one batch a week, with four seedlings intended for the system. Sow a few spares on top, as many as your own germination record says you need (Seeds, germination and propagation covers germination rates).
2. Nursery space. Each batch stays two weeks, so two batches are in the nursery at any time: 4 × 2 = 8 cells, plus spares.
3. System positions. Positions needed = heads per week × weeks in the system, counting a part week as a whole week, because a batch that stays 24 days is still there when the fourth batch after it arrives. At Cornell’s 24 days that is 4 × 4 = 16 positions; at Kratky’s 5 weeks, 4 × 5 = 20 positions.
4. Size for the slow season. Build for 20 positions. Under Cornell’s conditions the plants finish in 24 days¹, only 16 positions are busy, and the harvest is still four a week, because the harvest rate is set by how many you sow, not by how many positions you have.
5. Start-up lag. The first heads are ready 11 + 24 = 35 days, or 5 weeks, after the first sowing at Cornell’s pace¹, or about 2 + 5 = 7 weeks at Kratky’s². From then on you harvest four a week for as long as you keep sowing four a week.

The same arithmetic warns you about gaps. If one weekly sowing fails, the household is short of lettuce 5 to 7 weeks later (our arithmetic), when that batch should have been harvested. Kratky’s growers avoid empty space by harvesting and replanting on the same day, so the tanks stay at full capacity².

Long crops are planned around their harvest season instead. In Resh’s three-crop cucumber schedule, the next crop is sown 16 to 24 days before the old crop is pulled out, so young plants are ready when the space frees up (our arithmetic from⁴). The article Growing fruiting crops covers how to manage those months of harvest.

Recording your own timelines

Published timelines come from someone else’s climate, light and cultivar. After a few batches in your own system, your records are a better guide. For each batch, write down the cultivar, the sowing date, the transplant date, the date of first harvest, the date of last harvest for long crops, and the size or weight you harvested at.

Add the mean daily temperature, from a minimum–maximum thermometer read once a day, and a note on light: the season, or the hours your lamps ran. With those you can add up degree days for each batch using the thermal-time formula¹². If a winter batch took longer but collected a similar number of degree days as a summer batch, temperature explains the difference; if not, look at light (our suggestion, following the mechanism in¹²). Over a year this gives you a local table with your own ranges, which is the thing this article cannot give you.

Key numbers

Common mistakes

1. Using the packet’s “days to maturity” as an indoor calendar. It may be a field figure (this pepper’s counts from transplanting outdoors), and for transplanted crops it does not include the seedling weeks¹¹.
2. Assuming warmer is always faster. In the Brazilian lettuce study, the rate of development peaked at about 22 °C and fell above it¹³.
3. Sizing the system from the fastest timeline. Positions equal weekly harvest times weeks in the system, so a slow season needs more positions for the same harvest (our arithmetic from^1,2).
4. Treating time to first harvest as the length of a fruiting crop. Resh’s greenhouse tomatoes were first picked about 100 days after sowing and then harvested for about nine months (our arithmetic from⁴).
5. Comparing two timelines without comparing the harvest size. Kale from the same Cornell trial (a trade-magazine report) was ready as baby leaf after two weeks and at juvenile size after four⁸.

Summary

How long a crop takes depends on temperature, light, cultivar and the size you harvest at, so a useful timeline is a range with its conditions. Thermal time adds up daily temperature above a base temperature, and it explains why a cool crop runs slower; above an optimum, more heat stops helping. A seed packet’s “days to maturity” is a field figure that may count from sowing or from transplant, so read its definition before using it. Leafy crops in the trials here spent one to three weeks as seedlings and two to five weeks in the system, while greenhouse tomato, cucumber and pepper took two to four and a half months to the first pick and were then harvested for months. To turn a timeline into a schedule, sow a fixed batch every week; positions in the system equal the weekly harvest times the weeks each plant stays there. Record your own dates and temperatures, because your local table will soon beat any published one.

Next in the series: Growing leafy greens and herbs.

References

1. Brechner, M., Both, A. J. and CEA Staff (2013). Hydroponic Lettuce Handbook. Cornell University Controlled Environment Agriculture Program, Ithaca, NY. https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/8824/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf
2. 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
3. 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. Appendix 1. https://www.fao.org/3/i4021e/i4021e.pdf
4. 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.
5. Mason, J. Commercial Hydroponics, 3rd ed. (e-book). ACS Distance Education.
6. Walters, K. J., Tarr, S. and Lopez, R. G. (2023). Modeling purple basil, sage, spearmint, and sweet basil responses to daily light integral and mean daily temperature. PLOS ONE, 18(11), e0294905. https://doi.org/10.1371/journal.pone.0294905
7. Ferrarezi, R. S. and Qin, K. (2024). How to grow greenhouse spinach during Georgia summers using hydroponics. University of Georgia Extension Circular 1304. https://fieldreport.caes.uga.edu/publications/C1304/how-to-grow-greenhouse-spinach-during-georgia-summers-using-hydroponics/
8. Gagne, C., Mattson, N. S. and Holley, J. (2020). 5 top takeaways from trial of hydroponic kale cultivars. Greenhouse Grower, 7 October 2020 (trade magazine). https://www.greenhousegrower.com/?p=151275
9. Park, Y., Sethi, R. and Temnyk, S. (2023). Growth, flowering, and fruit production of strawberry 'Albion' in response to photoperiod and photosynthetic photon flux density of sole-source lighting. Plants, 12(4), 731. https://doi.org/10.3390/plants12040731
10. Treadwell, D. D., Hochmuth, R., Landrum, L. and Laughlin, W. (2010). Microgreens: a new specialty crop. University of Florida IFAS Extension, publication HS1164. https://journals.flvc.org/edis/article/download/118552/116475/173721
11. Johnny's Selected Seeds. "Days to Maturity" definition and product page for Sonoma (F1) bell pepper. Accessed 20 October 2026. https://www.johnnyseeds.com/vegetables/peppers/sweet-bell-peppers/sonoma-f1-bell-pepper-seed-5910.html
12. Castilla, N. (2013). Greenhouse Technology and Management, 2nd ed. CABI, Wallingford. Appendix A.2.1.
13. Brunini, O., Lisbão, R. S., Bernardi, J. B., Fornasier, J. B. and Pedro Júnior, M. J. (1976). Temperatura-base para alface cultivar "White Boston", em um sistema de unidades térmicas. Bragantia, 35(1), 213–219. https://doi.org/10.1590/S0006-87051976000100019

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.