When does cannabis flower outdoors?
If you grow cannabis outdoors, one question is crucial: when does cannabis flowers outdoors? The answer depends on various factors, such as the type of plant, the length of the days and your location. But in fact, the same applies to every strain: as soon as the plant experiences approximately 12 hours of darkness per day, it will start to flower. However, there are exceptions.
In this article, you will learn when cannabis plants start flowering, how to recognise this and what you can do to accelerate or optimise flowering.
Why is it important to know when cannabis flowers outdoors?
The flowering phase is when cannabis plants start to form their buds. It is a sensitive period in which nutrition, climate and care play a major role. If you don’t know when flowering starts, you run the risk of: 
- Harvesting too late → mould or bud rot in autumn.
- Harvesting too early → lower yield and less potent buds.
- Incorrect nutrition → growth nutrients during flowering can affect quality.
By knowing exactly when cannabis plants will start flowering outdoors, you can:
✔ Prevent mould by taking timely measures.
✔ Provide the right nutrition at the right time.
✔ Maximise your yield.
Photoperiod vs. Autoflower: big difference in flowering time
Before we go into more detail about the flowering time, we need to distinguish between photoperiod plants and autoflowers:
- Photoperiod (regular or feminised cannabis plants)These strains only flower when the days get shorter and the nights get longer. Outdoors, this usually happens from late July to mid-August.
- Autoflower (autoflowering cannabis plants)These strains flower independently of the number of hours of light. They start automatically, usually 3 to 5 weeks after germination, regardless of the season.
➡ Tip: If you want to be sure of a harvest before autumn, choose autoflowers.
How does the flowering phase of cannabis work outdoors?
Cannabis is a short-day plant. This means that it only starts flowering when the length of the night increases. So it’s not about how many hours of light the plant gets, but how many hours of darkness there are.
- In June, the days are longest: in the United Kingdom, approximately 16 to 17 hours of light.
- After 21 June (the longest day of the year), the days become shorter.
- As soon as the plant experiences approximately 12 hours of darkness per day, it will really start to flower.
In practice, this means that photoperiod cannabis plants only really start to flower outdoors at the end of July/August.
When cannabis flowers outdoors in the Netherlands and Belgium?
- Pre-flowering (first hairs): usually late July to early August.
- Full flowering (bud formation): from mid-August.
- Harvest: depending on the strain, late September to late October.
Indica vs. Sativa
- Indica-dominant strains flower faster (8-9 weeks) and are often ready by the end of September.
- Sativa-dominant strains take longer (10-14 weeks) and may not be ready until November. This is risky due to the wet autumn in the Netherlands and Belgium.
How can you tell when cannabis flowers outdoors?
The first signs of flowering are:
- Pre-flowersSmall white hairs (pistils) appear at the nodes of the plant.
- Stretch phaseYour plant experiences a growth spurt and can almost double in height in 2 weeks.
- Bud formationAfter 2-3 weeks, you will see compact flowering sites with more and more white hairs.
Factors that influence flowering
1. Latitude and day length
In the United Kingdom and Ireland, the days quickly become shorter after June. In southern Europe, the difference is smaller, which means that flowering sometimes starts later there.
2. The weather
A cold or wet summer can slow down growth, making flowering appear to start a little later.
3. Cultivation techniques
- Topping, LST or scrog can slow down growth, making flowering appear to start later.
- Growing indoors and then moving outdoors results in larger plants that are ready earlier.
4. Stress
Too much stress (such as repotting or pruning in July) can delay flowering.
Can you speed up flowering outdoors?
Yes, there are three ways to do this:
- Artificial darkeningFrom July onwards, give your plants 12 hours of complete darkness every day by covering them with a dark tent or cloth.
- Start sowing earlyStart germinating in April or May so that your plant is large and ready to flower as soon as the days get shorter.
- Growing autoflowers These strains flower automatically and are often ready for harvest in July or August.
What nutrients should you give during flowering?
During flowering, cannabis needs less nitrogen (N) but more phosphorus (P) and potassium (K).
- Start with a flowering fertiliser during pre-flowering.
- Add CalMag if necessary for strong buds.
- Do not give too much fertiliser at once to prevent burning.
Common mistakes during outdoor flowering
- Starting germination too late → small plants with low yield.
- No prevention against mould → botrytis (bud rot) is deadly for your harvest.
- Harvesting too early or too late → affects taste, effect and potency.
Autoflowers: a smart choice for outdoor growing
If you don’t want to stress about rain in October, choose autoflowers.
Advantages:
- Flower automatically, regardless of day length.
- Ready in 10 to 12 weeks after germination.
- Multiple harvests per season possible.
Conclusion: when cannabis flowers outdoors?
- Photoperiod plants usually start flowering at the end of July/August.
- Flowering is triggered by shorter days and longer nights.
- Autoflowers always flower on time, regardless of the light.
- In the United Kingdom and Ireland, the weather is a major factor, so plan wisely.
Frequently asked questions (FAQ)
1. How long does cannabis flower outdoors?
On average, 8-14 weeks, depending on the strain.
2. Can I make cannabis flower earlier outdoors?
Yes, by shading or using autoflowers.
3. When is the best time to harvest outdoors?
Usually between late September and late October.
Growing Cannabis Outdoors with Cannabis Seeds
How to Speed Up Cannabis Flowering Outdoor
Tips to Increase Your Cannabis Yield When Growing Outdoors
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