Granddaddy Purple Grow Time and Yield: Indoors and Outdoors

Short answer

Granddaddy Purple flowers in 8 to 11 weeks indoors. Most growers harvest at 9 or 10 weeks. Outdoor plants finish in late September or early October in the Northern Hemisphere. Indoor yield runs about 400 to 500 grams per square meter. Outdoor yield runs 500 to 900 grams per plant with full sun and large pots. Autoflower seeds finish 8 to 10 weeks after sprouting and yield 300 to 450 grams per square meter.

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Strain facts

• Genetics: Purple Urkle x Big Bud
• Type: indica-dominant, about 80% indica and 20% sativa
• THC: 17% to 23%
• Indoor height: 90 cm to 150 cm
• Aroma: grape, berry, sweet earth
• Effect: body-heavy and sedative, used in the evening

Indoor flowering timeline

Week 1 to 3: the plant stretches 50% to 100% in height under a 12/12 light cycle.

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Week 3 to 5: bud sites form at each node and calyx clusters stack along the branches.

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Week 6 to 9: buds swell and gain weight. Purple color shows on calyxes and leaf edges when night temps drop below 15°C (59°F).

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Week 9 to 11: trichomes shift from clear to cloudy to amber. Cut at 20% to 30% amber for a heavy effect. Cut at 5% amber for a milder head high.

Outdoor grow time

Start seeds indoors in early spring. Move plants outside after the last frost, once day length passes 14 hours. Vegetation runs through summer. Flowering starts in August as nights pass 12 hours. Harvest lands in late September to early October. Cold nights in the final 2 to 3 weeks bring out the purple.

Wet fall weather is the main risk. Dense colas hold moisture and rot. Check plants after each rain.

Vegetative time

Give GDP 4 to 6 weeks of vegetation indoors for a plant 90 cm to 120 cm tall. Add 2 more weeks for larger plants. The strain stretches hard at the flip, so leave 40 cm between the canopy and the light in week 1 of flower.

Yield numbers

Breeders list 400 to 500 grams per square meter indoors. Home growers with one 400W to 600W light over 1 square meter hit the lower end of that range. Outdoor plants in 30 to 50 gallon pots yield 500 to 900 grams. A trained plant in full sun can pass 1 kg (2.2 lb).

What raises yield

• Light: 600W HPS or 480W LED over 1 square meter.
• Training: low stress training and topping create 8 to 12 main colas.
• Defoliation: strip lower growth at week 3 of flower so energy goes to top buds.
• Pot size: 5 gallons minimum indoors, 30 gallons outdoors.
• Nutrients: hold nitrogen down in flower. Push phosphorus and potassium from week 4.
• Airflow: 2 fans plus an exhaust fan rated near 1 air change per minute.
• Room: 24°C to 28°C (75°F to 82°F) and 45% to 55% humidity in mid flower.

Purple color

Two triggers cause purple calyxes. Genetics come first. Cool nights of 10°C to 15°C (50°F to 59°F) in the last 2 to 3 weeks let anthocyanin pigment build. Warm rooms stay green even in purple genetics. Cold below 10°C slows growth and invites mold, so drop night temps in late flower only.

Problems to watch

• Bud rot: dense colas plus humidity above 60%. Hold humidity under 55% in late flower.
• Nutrient burn: dark green leaf tips curl down. Flush and cut feed by 25%.
• Spider mites: check leaf undersides each week. Treat with neem oil or insecticidal soap.
• Stretch: the plant can double in height over 3 weeks. Top early or add a trellis net.

Harvest and cure

Dry whole branches in the dark at 18°C to 21°C (65°F to 70°F) and 55% to 60% humidity. Small stems snap at 7 to 10 days. Trim, then cure in glass jars for 2 to 4 weeks. Burp jars for 10 minutes each day in week 1, then every 3 days after that.

Autoflower version

Granddaddy Purple auto seeds ignore the light schedule. Plants finish 8 to 10 weeks from sprout. Yield sits lower, about 300 to 450 grams per square meter, and plants stay near 60 cm to 90 cm. Purple color is less consistent in auto versions.