Loose-leaf lettuce / Salad bowl lettuce (Lactuca sativa var. crispa) growing guide

Quick facts

Category: leafy greens
Difficulty: beginner
Days to harvest: 30 to 50 days
Harvest type: cut leaves, plant regrows for repeated harvests
Spacing: 20 cm between plants

Environment

Temperature: 13–22°C
pH: 5.5 to 6.5
EC (hydroponic): 0.7 to 1.3 mS/cm
Daily light: 10 to 15 mol/m²/day

Climate and zones

USDA zones: 4 to 9 (winter low around -34°C or warmer)
Frost tolerance: tolerates light frost
Season: cool (spring and fall crops)

Viable growing environments:

outdoor year-round (in zone)
outdoor in growing season (annual)
unheated greenhouse / hoop house
heated greenhouse
indoor (heated home)
indoor hydroponics under grow lights

USDA zone bounds reflect outdoor year-round survival. Anywhere outside the bounded zone range, this crop still grows as an annual in the warm months (outdoor_seasonal), under cover (greenhouse), or indoors under lights.

Growing systems

Loose-leaf lettuce works in:

deep water culture (rafts)
NFT channels
vertical / aeroponic tower
media bed (ebb and flow)
wicking bed
soil bed

Growing media

The substrate the roots sit in. Choice depends on the system (clay pebbles don't fit NFT channels; rockwool isn't used in media beds) and the crop (loose-leaf lettuce works in the media listed below).

Medium	pH effect	Water retention	Bacterial surface
Rockwool (Mineral wool)	alkaline until pre-soaked	very high	low
Expanded clay pebbles (LECA)	neutral / inert	low	high
Coco coir (Coconut coir)	slightly acidic	high	moderate
Net pot, no medium (Bare-root)	-	-	-
Soil-based mix (Potting soil)	varies by source	high	high

Bacterial surface area matters for aquaponics: clay pebbles, lava rock, and pumice double as biofilter substrate. Low-surface media (rockwool, perlite, pea gravel) work in hydroponics but need a separate biofilter in aquaponics.

Nutrient demand by stage

NPK ratios are relative weights at each growth stage; the nutrient mix calculator scales them to absolute grams or ml. EC targets shift through the plant's life: seedlings need a much lighter solution than fruiting adults.

Stage	N	P	K	EC target (mS/cm)
seedling	1	1	1	0.6
vegetative	3	1	2	1.1

Aquaponics suitability

Compatible with typical aquaponics nutrient profiles. Fish waste provides enough nitrogen for healthy growth; supplemental potassium, calcium, and iron may still be needed depending on fish stocking density.

Care notes

The default hydroponic lettuce type and one of the most commonly grown hydroponic crops worldwide. EC 0.8-1.5 mS/cm. pH 5.5-6.5. Temperature: 15–22°C ideal (loose-leaf types tolerate 24–26°C slightly better than heading types before bolting, though summer heat remains the primary challenge). Moderate light (DLI 12-17 mol/m2/day; high DLI above 20 causes bitterness and bolting). From seed to baby leaf harvest: 3-4 weeks. Full-sized harvest: 5-6 weeks. NFT, DWC, raft, Kratky, and vertical tower systems all work. Cut-and-come-again harvesting (taking outer leaves while the center continues growing) extends each planting's productive life to 4-6 weeks of continuous harvest. Succession plant weekly for uninterrupted supply. Red varieties need slightly more light than green types to develop full color. Tipburn is less common in loose-leaf types than in heading lettuce because airflow reaches the inner leaves. For commercial growers, a mix of red, green, and specialty varieties allows premium salad mix pricing ($8-15/kg retail). Loose-leaf lettuce is the best crop for beginners learning hydroponic growing.

Notable varieties

A starting shortlist of cultivars worth knowing about. Not exhaustive: the seed catalogs list hundreds of named varieties. These are the ones home growers commonly choose between.

Cultivar	Type	Days	Notes
Black Seeded Simpson	heirloom	45	Pre-1850 heirloom, light green frilly leaves. One of the earliest spring lettuces. Bolts quickly in heat; cool-season only in zone 7+. Cut-and-come-again productive.
Red Sails	open-pollinated	55	1985 All-America Selections winner. Deep red-bronze ruffled leaves. Slower to bolt than Black Seeded Simpson, useful for stretching the season into early summer. Mild flavor, beautiful in mixed salads.
Salad Bowl	open-pollinated	50	1952 AAS winner. Light green deeply lobed oak-leaf-shaped leaves. Slow bolting in heat. Cut-and-come-again productive over 8-10 weeks if temperatures stay below 25C.
Lollo Rossa	open-pollinated	55	Italian frilly loose-leaf with intensely curled magenta-edged leaves. Slightly bitter flavor balances milder lettuces in a mix. Heat-tolerant compared to most loose-leafs.