Hydroponic lettuce: pH, PPM, and days to harvest
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Hydroponic lettuce is one of the most forgiving crops for learning reservoirs: short cycles, modest nutrient strength, and clear feedback when pH, heat, or concentration drifts. You still need real numbers — pH, PPM/EC, temperature, and days to harvest — not miracle “cut-and-come-again forever” promises.
This guide covers target specs with units, Kratky vs DWC for lettuce, a simple schedule, tip burn and bitterness fixes, cost notes, when to skip fancy systems, and FAQ. Cross-links: Kratky method · Hydroponic nutrients · Hydroponic system cost · DWC · Beginners · NFT
Shop: Check current lettuce-friendly nutrients, net pots, and Kratky/DWC parts on Amazon (paid link; disclosure)
What “hydroponic lettuce” means here
Leaf and butterhead types in DWC, Kratky, NFT, towers, and countertop kits. Crisphead/iceberg can work but is fussier on space and timing for beginners. We focus on leaf and loose-leaf first — faster edible returns.
Who it’s for: Beginners and hobbyists who want salad greens indoors or in a garage grow with measured solution.
Who should skip: Anyone expecting outdoor-bed volumes from one mason jar, or fruiting crops disguised as “lettuce practice.”
Spec table: pH, PPM, EC, temp, days
Ranges below are hobby starting points many growers use for leafy greens. Always confirm against your nutrient bottle chart and your meter’s scale (500 vs 700 PPM conversion from EC).
| Spec | Seedling / early | Established leaf lettuce | Notes |
|---|---|---|---|
| pH | ~5.5–6.5 | ~5.5–6.5 | Drift outside band → lockout symptoms |
| PPM (500 scale, typical) | ~400–600 | ~700–1,200 | Lower if tip burn; know your scale |
| EC (approx.) | ~0.8–1.2 mS/cm | ~1.2–2.0 mS/cm | Convert with your meter docs |
| Solution temp | ~65–72°F (18–22°C) | same | Warmer → less dissolved O₂, more rot risk |
| Air temp (rough) | ~60–70°F nights help quality | avoid sustained heat | Heat drives bitterness/bolting |
| Light (indoors) | ~14–16 h photoperiod common | same | Weak light → stretch, pale leaves |
| Days to harvest | — | often ~28–45 days from transplant for leaf types | Variety + light dependent; seed-to-harvest longer |
EC ↔ PPM reminder: If your meter uses a 0.5 factor, EC 1.4 ≈ 700 PPM; a 0.7 factor reads higher for the same EC. Compare apples to apples.
Kratky vs DWC for lettuce
| Factor | Kratky | DWC |
|---|---|---|
| Oxygen | Air gap as level drops | Continuous air stone |
| Noise | Silent | Pump hum |
| Maintenance | Low mid-cycle if sized right | Check air + chemistry more often |
| Heat tolerance | Poor if stagnant & warm | Better O₂ if pump strong — still hates heat |
| Cost to start | Very low per jar | Low–mid (pump added) |
| Best use | 1–N quiet lettuce sites | Multi-site buckets/totes, faster recoveries |
| Failure mode | Gap drowned or jar too small | Stone dies / power outage / dirty warm water |
Pick Kratky if you want quiet and cheap first heads — full method: Kratky.
Pick DWC if you want active aeration and larger shared reservoirs — DWC.
NFT fits when you want many sites in a line and will babysit flow — NFT.
Step schedule (transplant → harvest)
- Germinate in rockwool/plugs; keep moist, not swampy.
- Transplant when roots show; net pot into Kratky (initial wick contact) or DWC (roots toward solution + air on).
- Start mild nutrient strength; pH ~5.5–6.5.
- Week 1–2: stabilize light height; log pH/PPM 2–3× weekly.
- Week 3–4+: raise strength toward established band if plants are hungry and tips look healthy.
- Harvest whole head or outer leaves; don’t wait for heat-bitter giants.
Mixing order and bottle types: hydroponic nutrients. Flora-class ratios: General Hydroponics Flora Series.
Shop: Check current hydro nutrients and lettuce net-pot kits on Amazon (paid link; disclosure)
Tip burn: diagnose and fix
What you see: Brown crispy edges on young leaf margins, often newest growth.
| Likely cause | Why | Fix direction |
|---|---|---|
| Nutrient strength too high | Salt stress at leaf margins | Lower PPM/EC; top with pH’d water; remake reservoir if extreme |
| Calcium delivery issues | Ca mobility + transpiration balance | Steady environment; don’t swing lights hard; check Ca in formula / water |
| Low humidity + high light / VPD swings | Uneven uptake | Moderate extremes; don’t bake plants under a hot fixture |
| Root problems limiting uptake | Damaged roots can’t move Ca | Inspect roots; fix O₂/temp — root rot |
| pH out of band | Uptake stalls | Correct to ~5.5–6.5 |
Tip burn is not a reason to dump “more bloom booster.” Lower stress first.
Bitterness and bolting
| Issue | Trigger | Direction |
|---|---|---|
| Bitter leaves | Heat, long stress, late harvest | Cooler nights; harvest earlier; choose heat-tolerant leaf varieties |
| Bolting (tall flower stalk) | Heat + long day stress | Harvest ASAP; start next cycle cooler |
| Soft watery leaves | Low light, excessive N | Improve light; don’t chase dark green with endless nitrogen |
Taste is a quality metric. Grocery-perfect texture needs environment, not a special ASIN.
Variety and density notes
- Loose-leaf / oakleaf / butterhead: beginner favorites.
- Spacing: overcrowded net pots compete for light — thin or use more sites.
- Succession: start a new jar weekly for continuous salads instead of one giant gamble.
Cost notes (lettuce-specific)
| Item | Band (USD) | Note |
|---|---|---|
| Single Kratky site parts | ~$5–25 | Before shared light/meters |
| DWC 1–4 site starter | ~$40–150 | Pump included class |
| Nutrients share per head | cents–low dollars | Amortized from bottles |
| Electricity | low for air pump; lights dominate | See system cost |
No fake “ROI %.” Grocery comparison depends on your power rate, light hours, and whether you count labor.
When to skip hydro lettuce setups
Skip a multi-tier NFT build if you only want two salads a week — jars or one DWC bucket are enough. Skip if your room routinely runs hot and you can’t cool solution. Skip if you refuse to measure pH.
FAQ
What pH for hydroponic lettuce?
Most hobby growers target roughly 5.5–6.5. Measure; don’t guess.
What PPM for hydroponic lettuce?
Seedlings often ~400–600 PPM; established leaf lettuce often ~700–1,200 PPM on a common 500-scale meter — verify scale and bottle chart.
How long does hydroponic lettuce take?
From transplant, many leaf types land around ~28–45 days under decent indoor light. Seed-to-harvest is longer. Varieties differ.
Is Kratky good for lettuce?
Yes — classic use case. See Kratky method.
Why is my hydroponic lettuce bitter?
Heat and late harvest are common. Cool the space if you can; harvest earlier; reduce stress.
What causes tip burn on hydro lettuce?
Often high nutrient strength, environmental swings affecting calcium delivery, or root issues. Lower PPM and stabilize environment before adding products.
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Grow Bench on hydroponicsystem.com is an independent reference site. We are not a retailer or manufacturer. Cost bands are typical US hobby price ranges, not quotes. No yield guarantees, medical claims, or invented lab tests. See our affiliate disclosure.