Purple Auto Strain Review: Flowering Window, Yields, and Growing Tips

Most growers chasing purple genetics make the same mistake: they pick a photoperiod purple strain, start late, and watch October frost end their season before the colour even develops. Purple Auto solves that problem entirely. This strain finishes in 70 to 80 days from seed, ignores your light schedule, and produces the vivid anthocyanin-driven colour that purple hunters want , without the calendar risk that kills late-finishing photoperiod runs. If you're growing outdoors anywhere in Canada's Zones 3 to 6, or running an indoor tent with limited cycle time, this is the review that tells you exactly what to expect.

What is Purple Auto?

Purple Auto is an indica-dominant autoflowering cannabis strain selected for its combination of anthocyanin-rich purple colouration, compact plant structure, and fast seed-to-harvest cycle.

The purple pigmentation comes from anthocyanins , the same flavonoid compounds responsible for blueberry and red cabbage colour. Cooler overnight temperatures (below 15°C) during the last two weeks of flower trigger stronger purple expression. Indoors, growers can simulate this with a brief temperature drop during the dark cycle.

The autoflowering trait , inherited from Cannabis ruderalis genetics , means Purple Auto flowers based on age rather than photoperiod. Under autoflowering seeds conditions, plants begin transitioning to flower at roughly 21 to 28 days from germination regardless of how many hours of light they receive per day.

What is Purple Auto's Flowering Window and Full Cycle Timeline?

Purple Auto transitions to flower at approximately 21 to 28 days after germination and completes flowering in 45 to 55 additional days, putting the total seed-to-harvest window at 70 to 80 days.

Here's how those days break down in practice:

• Germination: Days 1 to 5 (paper towel or direct soil, 22 to 25°C)
• Seedling stage: Days 5 to 14 (first true leaves, minimal water)
• Vegetative stage: Days 14 to 28 (rapid node development)
• Pre-flower / stretch: Days 28 to 35 (height increase of 30 to 50%)
• Peak flower: Days 35 to 65 (bud site stacking, trichome development)
• Final ripening: Days 65 to 80 (trichome amber, flush or feed-out)

Most first-time growers underestimate the stretch phase. Between days 28 and 35, Purple Auto can increase in height by 30 to 50% , plan your tent height accordingly. Plants typically finish between 60 and 90 cm indoors.

Indoor Yields: What Can You Realistically Expect?

Indoor yields for Purple Auto range from 400 to 550 g/m² under optimized 18/6 LED conditions at 600 to 800 PPFD. Aggregated grower journals show that 11 L fabric pots typically produce 90 to 120 g per plant, while 19 L pots push that to 140 to 170 g per plant under the same schedule.

The biggest yield lever indoors is light intensity. Running below 500 PPFD consistently drops yields by 20 to 30% compared to 700 to 800 PPFD. A quality LED quantum board covering a 1.2 x 1.2 m tent at the correct hanging height (40 to 60 cm above canopy) is the single biggest return-on-investment for indoor Purple Auto production.

Optimal environmental parameters for indoor runs:

• Temperature (day/night): 22 to 26°C / 18 to 21°C
• Relative humidity (veg): 55 to 65% RH
• Relative humidity (flower): 40 to 50% RH
• CO₂: 800 to 1,200 ppm (if running sealed room)
• EC feed (peak flower): 1.4 to 1.8 mS/cm
• Light schedule: 18/6 (most common) or 20/4

Under feminized cannabis seeds conditions , meaning no males to cull , you can run 4 plants per 1.2 x 1.2 m space comfortably within the 4-plant household limit set by the Cannabis Act 2018.

Outdoor Yields Across Canadian Climate Zones

Outdoor yields for Purple Auto are naturally lower than controlled indoor environments, but the time-efficiency more than compensates. Grower-reported data puts per-plant outdoor yields at 50 to 120 g per plant in containers, with larger in-ground grows sometimes reaching 150 g under ideal sun exposure.

The 70 to 80 day cycle means a June 1 start produces a harvest by mid to late August , well ahead of first frost across nearly every Canadian province. A second run started in late June or early July can still finish before September frost in Zones 4 to 6.

Key outdoor yield factors for Canadian climates:

• Full-sun positioning (6+ hours direct light per day minimum)
• 20 to 25 L containers for outdoor unrestricted root zone
• Starting indoors 10 to 14 days before outdoor transplant hardening
• Monitoring overnight temps , keep plants covered if nights drop below 10°C during flower

The 14-Day Buffer Rule for Canadian Outdoor Growers

How to Grow Purple Auto: Step-by-Step

Step 1: Germinate in a warm, dark environment

Paper-towel germination at 22 to 25°C produces 90 to 97% success within 48 to 72 hours across grower journals and breeder documentation. Keep seeds moist but not soaking , excess water drowns the taproot before it anchors.

Step 2: Plant directly into your final container

Autoflowers dislike transplanting. Because the clock starts at germination, any transplant stress cuts into vegetative time. Plant directly into an 11 to 19 L fabric pot with a light, aerated mix (coco/perlite or peat-based with 30% perlite).

Step 3: Keep it light on water during seedling phase (Days 5 to 14)

Most growers over-water in week 2. The roots are still small , water in a ring around the seedling, not directly on the stem, and let the medium dry almost fully before watering again. Damping off is the most common early kill.

Step 4: Begin light feeding in week 3

Start nutrients at 50% of recommended dose (EC 0.8 to 1.0 mS/cm) once the third node appears. Ramp to EC 1.4 to 1.8 mS/cm by week 5. Autoflowers are sensitive to overfeeding , nutrient burn on autos is extremely difficult to reverse mid-cycle.

Step 5: Drop humidity below 50% in the final three weeks

Dense Purple Auto colas are botrytis-prone above 55% RH during late flower. Run a dehumidifier, increase airflow, and lightly defoliate the lower 30% of the canopy by week 6. Trichome check with a jeweller's loupe at week 9 to 10 , harvest when 10 to 20% of heads are amber for a balanced effect.

Common Mistakes Growers Make with Purple Auto

Mistake 1: Transplanting mid-cycle

Autoflowers run on a fixed internal clock. A transplant at week 2 sets the plant back 5 to 7 days of recovery , days it can't recover in a 70-80 day cycle. Start in your final pot. Every time.

Mistake 2: Running 12/12 light to "force" flower

Purple Auto flowers by age, not photoperiod. Dropping to 12/12 doesn't speed up flowering , it just cuts your daily light hours and reduces photosynthesis. Keep it at 18/6 or 20/4 all the way through.

Mistake 3: Expecting maximum purple without a temperature drop

Anthocyanin expression intensifies with cooler temperatures. Growers keeping their tent at 26°C night and day will see less purple than those allowing nights to cool to 17 to 18°C during the final two weeks. The genetics are there , but temperature activates them.

Mistake 4: Harvesting too early

Looking at public grower forum data, premature harvest is the most-cited Purple Auto disappointment. Growers see purple calyxes at day 65 and pull early. Wait for trichomes , not colour. Harvest at 10 to 20% amber trichome heads for peak potency.

Mistake 5: Starting outdoors in Canada after July 10

After July 10, light hours begin declining and overnight temps in most provinces start trending downward. A July 10 start puts harvest at late September to early October in an 80-day strain , dangerously close to first frost across Zones 3 to 5.

City-by-City Canadian Outdoor Grow Scenarios

Using Environment and Climate Change Canada climate normals and Plant Hardiness Zones of Canada data, here's how a June 1 Purple Auto start plays out across five major Canadian cities:

Across every major Canadian city tested, a June 1 Purple Auto start clears first frost with comfortable margins. Even the most frost-prone Zone 3 city in this dataset , Saskatoon , shows a 15 to 30 day buffer with the 80-day finish scenario.

Myth vs. Reality: Purple Cannabis Genetics

Royal King Seeds Purple Auto Risk Rating

Purple Auto vs. Photoperiod Purple Strains: Which Should You Choose?

This is the core decision most Canadian growers face. Here's a direct side-by-side:

For growers who want purple genetics without calendar risk, autoflower seeds in Canada are the practical choice. For experienced indoor-only growers prioritizing maximum yield per cycle, indica seeds in photoperiod form offer a higher ceiling.

Patterns From Aggregating Public Purple Auto Grower Reports

Looking at the questions our team fields most often during the grow season, and reviewing publicly available grower journal threads, a few consistent patterns emerge across Purple Auto runs in Canada.

The colour timing question spikes every August. Growers start messaging around week 7 asking why their plant isn't fully purple yet. The answer is consistent: purple expression peaks in the final two to three weeks of flower, often after a cooler night. Patience and a slight temperature drop are the reliable triggers.

Mould complaints cluster around week 9 to 10 in humid climates. This pattern appears most in Atlantic Canada and coastal BC grower reports , almost always tied to high overnight humidity and inadequate airflow around the canopy. The fix is consistent: defoliate the lower third by week 6 and position a fan to move air across the top of the canopy.

Yield under-reports are almost always a lighting issue indoors. Growers reporting 60 to 80 g per plant consistently show tent setups with insufficient PPFD , often 300 to 400 PPFD vs. the recommended 600 to 800. Upgrading to a proper quantum board LED is the single biggest yield lever for indoor Purple Auto runs. Check out our full germination and growing guide for environment setup basics.

Purple Auto FAQ

Sources

• Plant Hardiness Zones of Canada , Natural Resources Canada
• Environment and Climate Change Canada , Climate Normals
• Cannabis Act 2018 , Government of Canada
• Health Canada , Cannabis Regulation and Information
• Journal of Cannabis Research , BioMed Central
• UBC Faculty of Land and Food Systems , Crop Science Resources