Hydroponic Lettuce Seed to Harvest: EC, pH, Timing, Spacing

AFTER THIS CHAPTER

Run lettuce from sowing to a transplant-ready plug on a published timetable.
Hold EC, pH and solution temperature inside the cited lettuce ranges, and know why they differ.
Plan harvest cohorts from published weeks-from-transplant figures and cool the crop after cutting.

01 / Lettuce types and what each asks of you

Four categories, not a ranking: leaf lettuce grows loose leaves, butterhead forms a soft head, romaine grows upright and semi-head sits between. CTAHR VC-1 covers leafy, romaine and semi-head lettuce and notes that leafy cultivars tend to resist tip burn better than semi-head and head lettuces. Cornell’s CEA lettuce handbook is built around 150 g (5 oz) heads of leaf and Boston bibb lettuce.

The type decides the harvest. University of Minnesota Extension says leaf lettuce can be cut repeatedly, lowest mature leaves first, most growers taking 3–5 harvests before reseeding, while head lettuce is harvested once and will not resprout. UF/IFAS HS1422 asks Florida growers to plant a cultivar adapted to shorter day length and warmer conditions: choose for your season. The leafy-greens chapter holds the general plan; this one stays with lettuce.

Reading: A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce (VC-1) ↗ · Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗ · Small-scale hydroponics ↗ · Growing Lettuce in Small Hydroponic Systems (HS1422) ↗

02 / Germination and the nursery

UF/IFAS HS1488 soaks grow cubes, sows one or two seeds per cube, holds them in a covered container with a little water and high humidity, and expects transplants after one to three weeks. VC-1 warns that lettuce seed suffers thermodormancy at temperatures as low as 82°F; its remedy is to cover freshly seeded trays in a cool place for 24 hours.

Cornell’s nursery runs on a clock: 20°C (68°F) for the first 24 hours under 50 µmol/m²/s with humidity covers; from Day 1, fertilizer at 1,200 µS/cm above source water at pH 5.8, 25°C and 250 µmol/m²/s on a 24-hour photoperiod; covers off Day 2; doubles removed Days 3–4. VC-1 says a seedling transplanted at one week needs no fertilizer, but seedlings kept longer get a quarter to half the production rate. Two programs’ settings, not a recipe.

Reading: “Set It and Forget It” Hydroponic Lettuce (HS1488) ↗ · A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce (VC-1) ↗ · Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗

03 / Transplant size, timing and spacing

The cue is the root, not the date. UMN says cubes are ready when roots have pushed through the bottom. Cornell transplants on Day 11, when roots have grown through the plug tray and leaves begin to overlap. VC-1 transplants 1–3-week-old seedlings with the lower ½–1 inch of the net pot immersed so the medium wets by capillary action.

Spacing is settled before pots are bought; the net-pot guide works through the layouts. Carry two figures: VC-1’s 1.5 and 1.9 plants per square foot, with its warning that more than two crowds large head cultivars, and Cornell’s Day 21 re-spacing from 97 to 38 plants per square metre. Oregon State EM 9457 lists lettuce as a popular NFT crop for its fast growth and shallow roots.

Soak plugs and sow one or two seeds each (HS1488; VC-1); label cultivar and date.
Cover the trays: 24 hours in a cool place (VC-1), or humidity covers at 68°F under 50 µmol/m²/s with the covers off on Day 2 (Cornell).
Feed seedlings kept past a week: a quarter to half the production rate (VC-1), or 1,200 µS/cm from Day 1 (Cornell).
Remove doubles on Days 3–4 (Cornell); keep spare plugs for selection.
Transplant when roots show through the plug (UMN; Cornell Day 11; VC-1 at 1–3 weeks), plug touching solution.
Next day, check every position for wilt and a dry plug (seeds-to-transplants chapter).

Reading: Small-scale hydroponics ↗ · Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗ · A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce (VC-1) ↗ · Hydro hints: Nutrient film technique ↗ · “Set It and Forget It” Hydroponic Lettuce (HS1488) ↗

04 / Solution EC, pH, temperature and oxygen

HS1422 prints an ideal EC of 1.4–1.8 mS/cm with pH 6.0–7.0 in its nutrition section and 1.2–1.8 mS/cm in its further considerations; Oklahoma State’s EC and pH guide lists lettuce at 1.2–1.8 mS/cm and pH 6.0–7.0, checked daily. VC-1 starts near 1.5 mS, reports growers using 1.2–1.5 mS in hot weather and 1.6–2.0 mS in cool weather, says be wary above 2.0, and recommends pH 5.5–6.5. Cornell runs 1,200 µS/cm above source water at pH 5.8 (5.6–6.0 acceptable). Pick one program and log against it.

VC-1 notes meters read higher as solution warms, 1.28 mS at 68°F becoming 1.55 mS at 86°F, so read at the same hour. HS1422 keeps the solution between 65°F and 80°F with dissolved oxygen of 5 mg/L. Cornell holds pond water no higher than 25°C and says DO should stay above 4 ppm, with visible stress at 3.

Reading: Growing Lettuce in Small Hydroponic Systems (HS1422) ↗ · Electrical Conductivity and pH Guide for Hydroponics ↗ · A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce (VC-1) ↗ · Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗

05 / Air temperature, light, tip burn and bolting

VC-1 says lettuce grows best at 60–70°F and tolerates the 80s and low 90s at the cost of tip burning and bitter taste. Cornell’s set-points are 24°C day and 19°C night (75°F/65°F) at 50–70 percent relative humidity. Cornell treats tip burn as calcium transport: a paddle fan blowing air downward at 140 cubic feet per minute per square foot of pond let it run 17 mol/m²/d; without airflow it could not go over 12 mol/m²/d, a result it limits to cultivar Ostinata.

HS1422 says Florida cultivars are adapted to 10–11 hours of light a day and northern cultivars may need more; too many hours can stunt growth and distort leaves. A study abstract reproduced in the Cornell handbook held a 16-hour photoperiod to prevent early bolting. For pests and disease HS1422 relies on scouting and integrated management and refers growers to extension specialists; so does this chapter.

Reading: A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce (VC-1) ↗ · Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗ · Growing Lettuce in Small Hydroponic Systems (HS1422) ↗

06 / Harvest, cooling and the crop calendar

VC-1 says leafy or semi-head lettuce is ready about 4–6 weeks after transplanting, depending on transplant age, season (winter takes longer) and cultivar. Cornell harvests on Day 35 at about 150 g; UMN gives 6–7 weeks without saying whether that runs from sowing or transplant; HS1422 gives 50–60 days. VC-1 harvests in early morning with washed hands and a sanitized knife or scissors into new bags or sanitized containers; UMN adds a sanitized tote, keeping drip off neighboring plants and rinsing produce.

Cooling in plain terms: Cornell stores packaged lettuce at 40°F. Cut before the day warms, keep heads out of sun and refrigerate without delay. HS1488 cuts the whole head at the base; keeping it whole until it is used is original advice, not a published finding.

Stage	Published figure	Source
Transplant	Roots through the plug; Day 11; ½–1 inch of pot immersed	UMN; Cornell; VC-1
Grow-out	4–6 weeks from transplant; re-space Day 21 (97 → 38/m²); harvest Day 35	VC-1; Cornell
Seed to harvest	35 days from sowing; 6–7 weeks (start point unstated); 50–60 days	Cornell; UMN; HS1422
Post-harvest	Early-morning cut, sanitized tool, new bags; stored at 40°F	VC-1; Cornell

FIELD QUESTIONYou have 48 raft positions at VC-1’s 1.5 plants per square foot and want 12 whole heads every week. How many cohorts fit, how long can each occupy the raft, and when do you sow?

48 ÷ 12 = 4 cohorts of 12, so each holds its positions 4 weeks: the short end of VC-1’s 4–6 weeks from transplant. If your cultivar and season need 6 weeks, you need 6 × 12 = 72 positions, or you cut 48 ÷ 6 = 8 heads a week. Sow weekly, one nursery period ahead of each transplant: 11 days on Cornell’s schedule, 1–3 weeks by VC-1 and HS1488, so about two weeks. Sow about 15 plugs for 12 positions to allow selection (an original allowance).

ILLUSTRATED SYSTEM INVENTORY

Parts & buying criteria

Build your own parts & cost worksheet →

Showing nutrient film technique. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.

01Reservoir1 · illustrated quantity+

Holds the recirculating nutrient solution. This is the common sampling and refill point for both channels.

Inspect: In cutaway view, compare the water surface with the pump intake. The drawing leaves room above the water; it does not specify a working volume.

Maintain: Record the level before adding water. Inspect the cover, clean accessible deposits and keep the intake submerged during use.

BUYING CRITERIA

Opaque, cleanable container rated by its manufacturer for the intended use; accessible lid and drain.

Check size, materials and operating conditions with your chosen supplier before ordering.

02Pump & supply1 · illustrated quantity+

Lifts solution from the reservoir to the channel inlets. Cyan packets indicate the direction of delivery.

Inspect: Follow the rising supply pipe and its branch to each channel. Both inlets need an observable supply.

Maintain: Disconnect power before opening the pump. Inspect its inlet screen and confirm both branches flow after reassembly.

BUYING CRITERIA

Compare the pump curve at the installed lift and pipe resistance; include service access and replacement availability. Supply the pump, timer and any air pump through ground-fault protection (GFCI/RCD), keep plugs and cord connections above the water line with a drip loop, and switch off and unplug before reaching into the solution.

Check size, materials and operating conditions with your chosen supplier before ordering.

03Growing channels1 · illustrated quantity+

Contain the shallow stream beneath the plants and guide it toward the return.

Inspect: Open the cutaway to find the channel floor beneath the roots. The slope is exaggerated for reading.

Maintain: Look along each channel for ponding, debris or an obstructing root mat; inspect the outlet before changing the pump.

BUYING CRITERIA

Removable covers, supported slope, smooth cleanable surfaces and outlets accessible with plants in place.

Check size, materials and operating conditions with your chosen supplier before ordering.

04Plant baskets6 illustrated · illustrated quantity+

Bridge the opening in the channel cover while allowing roots to pass through.

Inspect: In exploded view, the baskets lift above the channels. The open slats reveal the relationship between support and roots.

Maintain: Check that each rim sits securely and that roots are not pinched at transplanting or inspection.

BUYING CRITERIA

Match the basket rim and hole diameter; allow removal without damaging adjacent plants.

Check size, materials and operating conditions with your chosen supplier before ordering.

05Support mediumFor 6 baskets · illustrated quantity+

The illustrated expanded clay supports the seedling in its basket. The channels are not filled with aggregate.

Inspect: Select the brown pebbles and separate the basket in exploded view. Media texture is simplified.

Maintain: Remove loose particles before installation. Inspect whether the starter plug stays appropriately moist.

BUYING CRITERIA

Select a stable grade that stays in the basket; compare water retention, dust and cleaning requirements.

Check size, materials and operating conditions with your chosen supplier before ordering.

06Gravity return1 · illustrated quantity+

Collects channel outflow and brings it back to the reservoir without a second pump.

Inspect: Trace the low ends of both channels into the common return. Inspect the full path before assuming the circuit is clear.

Maintain: Remove obstructions and check for leaks. Test drainage with the pump off during commissioning.

BUYING CRITERIA

Removable unions, supported pipe and sufficient drain capacity under the actual installation conditions.

Check size, materials and operating conditions with your chosen supplier before ordering.

SOURCES & EDITORIAL STATUS

Evidence beside the lesson.

Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.

Independent specialist review is pending.

Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.

University of Hawai‘i at Mānoa, CTAHR (B. A. Kratky)A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce (VC-1) ↗
Originating description of the suspended net-pot method: single initial fill, falling level and the expanding moist air space in which upper roots develop; tank covers of ½–1-inch expanded (2 lb/ft³) or extruded polystyrene handled as 2 × 4 ft pieces and painted white; 2-inch hole-saw technique; lettuce densities of 1.5 and 1.9 plants/ft² (48 and 60 per 4 × 8 ft tank) with staggered layouts; 1–3-week-old seedlings with the lower ½–1 inch of the net pot immersed; 10-percent bleach soak of net pots after harvest.
Cornell University CEA Program (M. Brechner and A. J. Both)Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗
Pond-stage lighting: supplemental 100–200 µmol/m²/s at plant level for a total of 17 mol/m²/d of natural plus supplemental light; the 17 mol/m²/d target having to be matched with downward airflow to prevent tip burn, and the statement that without that airflow the authors were not able to go over 12 mol/m²/d, with the caveat that the data are for cultivar Ostinata only.
University of Minnesota ExtensionSmall-scale hydroponics ↗
Root support, transplanting, light, aeration and cleaning.
University of Florida IFAS ExtensionGrowing Lettuce in Small Hydroponic Systems (HS1422) ↗
Lettuce system choice and management.
University of Florida IFAS Extension“Set It and Forget It” Hydroponic Lettuce (HS1488) ↗
A small non-circulating lettuce example; seasonal and product assumptions apply.
Oregon State University ExtensionHydro hints: Nutrient film technique ↗
Channel slope, root space and flow continuity.
Oklahoma State University ExtensionElectrical Conductivity and pH Guide for Hydroponics ↗
Water analysis, salinity, pH and monitoring.

CONTINUE LEARNING

The next useful connections.

Media & propagation

Browse all 52 field guides →

Hydroponic lettuce from seed to harvest: EC, pH, timing and spacing

01 / Lettuce types and what each asks of you

02 / Germination and the nursery

03 / Transplant size, timing and spacing

04 / Solution EC, pH, temperature and oxygen

05 / Air temperature, light, tip burn and bolting

06 / Harvest, cooling and the crop calendar

Parts & buying criteria

Evidence beside the lesson.

The next useful connections.

From seed to a working root system

Leafy greens and herbs: plan a repeatable crop

Kratky vs DWC for lettuce: pick by power, season and scale

NFT or DWC for leafy greens: flowing film or still bath

Choosing net pots, collars and plant spacing for NFT and rafts