What Is Rainbow Runts?
Rainbow Runts — more formally listed as Rainbow Runtz — is a balanced hybrid cannabis strain built primarily on a Zkittlez × Do-Si-Dos cross, with some cuts also incorporating Gelato #41 or RS11 depending on the breeder. The result is a cultivar that leans on Zkittlez for its signature fruit-forward terpene profile and vivid bud coloration, while Do-Si-Dos brings dense structure, deep resin production, and a grounding body effect. If you've grown either parent, you already know what you're working with: medium-height plants, tight internodal spacing in flower, and a nose that starts sweet and finishes earthy.
From a cultivation standpoint, Rainbow Runts is a grower-friendly strain — not a beginner's nightmare. It responds well to training, tolerates moderate feeding, and colors up beautifully under cooler late-flower temperatures. That said, its tight bud structure demands airflow discipline. Let humidity creep above 55% in week 6 of flower and you'll be trimming out botrytis. Keep the environment locked in and it rewards you with some of the most photogenic, potent harvests you'll pull.
Browse Rainbow Runtz grow journals on Grow Guide to see how other growers are running this strain.
Rainbow Runts Grow Timeline at a Glance
Setting Up Your Environment for Rainbow Runts
Rainbow Runts is an indoor-friendly strain, and the data backs that up: of the 1,000 tracked grows on Grow Guide, 732 run indoors — for good reason. Controlled environments let you manipulate temperature and humidity at will, which is exactly what this strain needs in late flower to trigger its signature purple and blue pigmentation.
Temperature and Humidity Targets
- Vegetative stage: 72–82°F (22–28°C), 55–65% RH
- Early flower (weeks 1–4): 70–80°F (21–27°C), 45–55% RH
- Late flower (weeks 5–9): 65–75°F (18–24°C), 40–50% RH
- Final 2 weeks: Drop night temps to 60–65°F (15–18°C) to push anthocyanin expression and color development
VPD (Vapor Pressure Deficit) should sit between 0.8–1.0 kPa in veg and 1.0–1.4 kPa through flower. Running too high a VPD in early veg stresses young roots; too low in late flower invites mold into those tight bud clusters. Use a quality digital hygrometer-thermometer combo and check it twice daily until you know your space.
Lighting
Run 18/6 through veg and flip to 12/12 to trigger flowering. Full-spectrum LED is the go-to for Rainbow Runts — it runs cooler, allows tighter light-to-canopy distances, and modern quantum board fixtures hit the PPFD targets this strain likes without baking your tops.
- Vegetative PPFD: 400–600 µmol/m²/s
- Early flower PPFD: 600–800 µmol/m²/s
- Peak flower PPFD: 800–1,000 µmol/m²/s
Use our Grow Light Calculator to dial in your fixture height and coverage area before you start.
Training Rainbow Runts: Top Early, Train Wide
Left to grow naturally, Rainbow Runts tends toward a Christmas tree structure — one dominant main cola with smaller side branches. That's fine for small spaces, but if you want to maximize sites and even out your canopy, a combination of topping and LST (low-stress training) is the move.
Topping: Perform your first top at node 4–5, once the plant has established 4–6 nodes. This splits the apical meristem into two main colas and kickstarts lateral branching. Give the plant 5–7 days to recover before any further stress. A second top at node 6–7 two weeks later will push you toward a 4–8 cola structure depending on how hard you trained. For a deep dive on topping vs. other canopy techniques, see our Fimming vs Topping Cannabis guide.
LST: Start bending and tying main branches outward from week 3 of veg. The goal is a flat, even canopy — every top within 2–4 inches of the same height at flip. Rainbow Runts responds well to this; its branching is vigorous and flexible enough early on that you rarely snap anything with careful work.
Defoliation: Do a moderate defoliation at day 21 of flower (remove large fan leaves blocking bud sites) and again at day 42. Don't strip the plant — leave enough canopy for photosynthesis. Remove fans blocking lower bud sites and any growth you know won't make it to the canopy.
Feeding Rainbow Runts: NPK, EC, and pH
Rainbow Runts is a moderate-to-heavy feeder — not as hungry as some Gelato-heavy hybrids, but it appreciates a structured feed program. Most Grow Guide users run soil (633 of 1,000 tracked grows), which buffers pH well and forgives minor feed errors. Coco coir growers (148 of 1,000) need to be sharper on EC and pH since there's no buffering capacity.
Vegetative Feeding
- NPK target: Higher nitrogen during early veg — a 3:1:2 ratio works well
- EC range (coco/hydro): 1.2–1.8 mS/cm
- pH (soil): 6.2–6.8 | pH (coco/hydro): 5.8–6.2
Flowering Feeding
- Weeks 1–3 of flower: Transition — reduce N, increase P and K. EC 1.6–2.0 mS/cm
- Weeks 4–7 (peak bulk): Low N, high P/K — NPK approximately 2:4:6. EC 1.8–2.2 mS/cm
- Weeks 8–9 (final flush/fade): Plain water or minimal feed to allow natural senescence. EC under 0.6 mS/cm
Watch for magnesium deficiency in weeks 3–5 of flower — it's common on Rainbow Runts, showing as interveinal chlorosis on mid-canopy leaves. A foliar or root drench of 1–2 g/L of magnesium sulfate (Epsom salts) at the first sign usually resolves it within 5–7 days. If you're seeing other symptoms you can't identify, run them through the Nutrient Deficiency Identifier.
For seedling-stage nutrition, see our guide to best nutrients for cannabis seedlings. If you're going organic, our best organic nutrients guide covers compatible options for Rainbow Runts.
Harvesting Rainbow Runts
Rainbow Runts typically hits its harvest window at 56–63 days of flower (8–9 weeks). Don't harvest by the calendar — use trichome inspection as your primary trigger.
Trichome Reading
- All clear trichomes: Too early — THC hasn't peaked
- Mostly milky white, few amber: Peak THC — cerebral, energetic effects dominant
- 30–40% amber: THC degrading to CBN — heavier, more sedative effect
For most growers, the sweet spot on Rainbow Runts is 80–90% cloudy/milky with 10–20% amber on the sugar leaves and calyxes. Use a jeweler's loupe at 60–100x or a digital USB microscope. Check trichomes on the bud calyxes themselves, not the sugar leaves — leaves amber faster and will give you an inaccurate read.
Harvest in the morning before your lights come on (for indoor grows) when terpene and resin levels are at their highest. Use sterilized scissors and work quickly but gently — Rainbow Runts builds substantial trichome coverage and it's easy to knock resin heads off with rough handling.
Drying and Curing Rainbow Runts
The post-harvest phase is where most growers lose the quality they worked 14+ weeks to build. Rainbow Runts has a complex terpene profile — it needs a slow dry and a proper cure to develop fully.
Drying
- Temperature: 60–68°F (15–20°C)
- Humidity: 50–55% RH
- Airflow: Gentle indirect circulation — fans should not blow directly on buds
- Duration: 7–12 days. Hang whole branches or use drying racks
- Darkness: Keep the dry room dark — UV degrades terpenes
The bud is ready for jars when the smaller stems snap rather than bend and the outside of the bud feels dry to the touch, but the interior still has slight moisture. If you rush this below 5 days, you'll trap chlorophyll and the cure will taste harsh regardless of how long you run it.
Curing
- Place trimmed buds loosely into wide-mouth glass mason jars — fill to 75% capacity maximum
- Week 1: Burp jars 2–3 times per day for 15–20 minutes each session
- Weeks 2–3: Burp once daily
- Weeks 4–6: Burp every 2–3 days
- Target internal jar RH of 58–62% (use Boveda 62% packs to dial this in)
- Store sealed jars in a cool, dark location — a drawer or cupboard is ideal
Minimum cure for Rainbow Runts is 3 weeks. Six weeks is significantly better — the terp profile opens up and that characteristic sweet, tropical-fruit nose becomes pronounced. Don't shortcut this. Use the Dry & Cure Timer to track your drying and curing schedule automatically.
To estimate what your harvest might yield before you pull it, run your numbers through the Yield Calculator.
Common Rainbow Runts Problems and Fixes
| Problem | Likely Cause | Fix |
|---|---|---|
| Botrytis in dense colas | RH above 55% in late flower | Drop RH to 40–45%, increase airflow, defoliate around affected sites |
| Interveinal yellowing (mid-canopy) | Magnesium deficiency | 1–2 g/L Epsom salt drench or foliar spray |
| Slow lateral branching | Insufficient LST / late topping | Top at node 4–5 no later than week 3 of veg |
| Stretching at flip | Low PPFD or too much N at transition | Reduce N, increase light intensity, lower canopy distance |
| Pale color — no purple expression | Night temps too warm in late flower | Push night temps to 60–65°F (15–18°C) in final 2–3 weeks |
Tracking Your Rainbow Runts Grow
Keeping a grow diary is genuinely useful for a strain like Rainbow Runts — the variables (feed timing, defoliation days, temperature swings) directly affect outcome quality, and tracking them means you can replicate or improve your results next run. See our guide on how to keep a cannabis grow diary and the full cannabis grow diary 2026 guide for a structured approach.
Use the Grow Schedule Planner to map out your full Rainbow Runts timeline — from flip date through estimated harvest and cure completion.
Frequently Asked Questions
How long does Rainbow Runts take to flower?
Rainbow Runts typically finishes flowering in 56–63 days (8–9 weeks) from the 12/12 flip. Always confirm harvest readiness by inspecting trichomes — 80–90% cloudy with 10–20% amber on the calyxes is the target window for most growers.
What's the difference between Rainbow Runts and Rainbow Runtz?
"Rainbow Runts" is a common alternate spelling of Rainbow Runtz — they refer to the same strain. The Runtz branding (from the candy) is the original, but "Runts" is widely used in grow communities and seed listings.
Why isn't my Rainbow Runtz turning purple?
Purple and blue anthocyanin expression requires cool night temperatures — push nights down to 60–65°F (15–18°C) during the final 2–3 weeks of flower. If your grow space stays above 70°F at night, you'll get minimal color regardless of genetics.
Is Rainbow Runts suitable for beginner growers?
It's a manageable strain for growers with at least one grow under their belt. It's not particularly fragile, but its dense bud structure requires strict humidity control below 50% RH in late flower to avoid mold. Beginners comfortable with environment management will do fine.
How long should I cure Rainbow Runtz?
Minimum 3 weeks, but 6 weeks produces a noticeably better result — the sweet, tropical-fruit terpene profile develops and smooths out significantly with extended curing. Maintain 58–62% RH inside sealed glass jars and burp daily for the first week.
References
- Sirius Seeds. (2024). Rainbow Runtz Strain Profile — Genetics, Effects, and Cultivation Notes. Describes the Zkittlez × Do-Si-Dos lineage and balanced hybrid profile of Rainbow Runtz. sirius.nl
- Strainy.com. (2024). Rainbow Cannabis Strain — Grow and Harvest Guide. Covers light cycles, trichome harvest timing, and drying parameters including 60–70°F dry room temperatures. strainy.com
- Joint Commerce / Philosopher Seeds. (2024). Rainbow Runtz by Philosopher Seeds: A Comprehensive Strain Guide. Details flowering-stage environmental targets including 68–78°F temperature range and 40–50% RH for optimal resin production. app.jointcommerce.com
- Grow Guide Platform Data. (2026). Internal grow journal dataset — 1,000 tracked grows. Environment breakdown: 732 indoor, 172 outdoor; medium breakdown: 633 soil, 148 coco coir. Original data, Grow Guide.
- Caplan, D., Dixon, M., & Zheng, Y. (2017). Optimal Rate of Organic Carbon Fertilization and PPFD for Cannabis sativa Production. HortScience, 52(11), 1498–1504. Establishes evidence-based PPFD recommendations for cannabis cultivation applicable to strains including Rainbow Runtz. doi.org/10.21273/HORTSCI12363-17
