Short version? Most growers only need one PK shot and maybe one metabolic booster. For the PK, CANNA PK 13/14 is the classic, it's from £5.50 and you only add it once per reservoir. Want something working right through flowering? CANNA Boost Accelerator sits happily alongside any base feed and any PK. Advanced Nutrients users should look at Advanced Nutrients Big Bud for weeks 2 to 4. And if you grow organic in soil, BioBizz Top-Max is certified organic and costs pennies per feed. Whatever you pick, don't stack three of them in one tank. More on that below.
The best bloom boosters
CANNA PK 13/14
The PK shot most people mean when they say "PK". Concentrated phosphorus and potassium for the short stretch when flowers are building fastest, and that's all it tries to do.
- 1.5ml per litre
- Once per reservoir
- Any medium
- 250ml, 1L, 5L
- One dose per reservoir fill, roughly three weeks before harvest, for 3 to 6 days. A 250ml bottle goes a long way
- Works in soil, coco, hydro and run-to-waste
- Pairs with CANNA Boost if you want the full CANNA bloom set
CANNA Boost Accelerator
Not a PK at all. CANNA Boost speeds up the plant's metabolism so it feeds better and flowers more evenly. Different job, which is why it plays nicely with whatever PK you're already using.
- 2 to 4ml per litre
- No growth regulators
- Hard or soft water
- 250ml to 10L
- Use it from the moment flowers start forming right through to the finish
- Compatible with any base nutrient and any PK style booster, so you're not locked into one brand
- Standard rate is 2ml per litre. Push to 4ml if you want more flowering support
Advanced Nutrients Big Bud
A PK with extras. Big Bud adds L-amino acids, magnesium and organic acids on top of the phosphorus and potassium, and it's made for weeks 2 to 4 of flowering when the plant is putting down flower sites.
- 2ml per litre
- Weeks 2 to 4
- Hydro, coco, soil
- 250ml to 10L
- Magnesium is in there to help balance potassium uptake, which takes some of the guesswork out
- Slots straight into an Advanced Nutrients pH Perfect line-up
- Growing in coco? There's a coco-tuned COCO Big Bud too
Rock Resinator
A complex PK blend with soluble potash, organic acids and amino acids, built to pack size and weight on in late flower. It's one of the few that runs all the way through the flush.
- 1 to 3.5ml per litre
- Week 2 to harvest
- All media
- 250ml to 20L
- Start at 1 to 2ml per litre from week 2 and build up to around 3 to 3.5ml through the final weeks
- Comes in everything from a 250ml trial bottle to a 20L drum for bigger rooms
Want the full rundown? Read our closer look at Rock Resinator.
View Rock Resinator
House & Garden Top Booster
Bio-available phosphorus and potassium in a gel. Same active ingredients as House & Garden Shooting Powder, just without the faff of weighing out a powder.
- 1.5 to 3ml per litre
- Final four weeks
- Soil, coco, hydro
- 500ml, 1L, 5L
- Goes in once the flowers have set, over the final four weeks of bloom
- Works with any House & Garden base nutrient
BioBizz Top-Max
OMRI and SKAL certified organic. Top-Max speeds up cell division in the flowers and frees up calcium, iron and magnesium, rather than dumping a load of PK in.
- 1 to 3ml per litre
- Every watering
- Certified organic
- 250ml to 10L
- Simple routine. Every watering from week 2 of flower, stop a week before harvest
- Made for soil but safe in coco and hydro too
- The 250ml bottle is a cheap way to try it before you commit to a 5L
Using a bloom booster without overdoing it
Boosters sell on the promise of bigger flowers. Fair enough. But more isn't better here, and a tank full of three different PKs is how people end up with burnt tips and a plant that's stopped eating.
Your base bloom feed already has phosphorus and potassium in it. A PK booster tops that up for a short window. Push past what the plant can use and the extra just sits in the root zone, messing with uptake of everything else.
- Pick one PK. CANNA PK 13/14, Big Bud, Rock Resinator or Top Booster. One. They're doing the same broad job.
- A metabolic booster can go alongside. CANNA Boost and BioBizz Top-Max aren't PK shots, so they stack with one without doubling up.
- Stick to the timing on the bottle. CANNA PK is a few days near the end. Big Bud is weeks 2 to 4. Top Booster is the final four weeks. They're not interchangeable just because they're all "bloom".
- Start at the low end of the dose. Most of these give a range. Begin at the bottom, watch the plant, then nudge up.
- Check your numbers after mixing. A booster adds to your EC and can shift pH. Measure after it's in, not before. Our pH and EC meters guide covers the kit for that.
And if your plant's looking pale or patchy in flower, a booster won't fix it. That's usually a base feed or a CalMag problem. See our CalMag guide.
At a glance
Which one does what, and when it goes in.
| Booster | Type | When | Price |
|---|---|---|---|
 CANNA PK 13/14 |
PK shot | 3 to 6 days, about 3 weeks before harvest | From £5.50 |
 CANNA Boost Accelerator |
Metabolic booster | First flowers to finish | From £21.50 |
 Advanced Nutrients Big Bud |
PK plus amino acids | Weeks 2 to 4 | From £11.80 |
 Rock Resinator |
Complex PK | Week 2 to harvest | From £25.00 |
 House & Garden Top Booster |
PK gel | Final four weeks | From £9.00 |
 BioBizz Top-Max |
Organic stimulant | Week 2 to a week before harvest | From £8.09 |
Prices as at November 2026. Check the product page for the live price.
How to choose
- First grow, one tent? CANNA PK 13/14. Cheap, simple, and you only use it for a few days.
- Already on Advanced Nutrients? Big Bud for weeks 2 to 4, then Overdrive to finish.
- Want help through the whole of flower? CANNA Boost Accelerator, with one PK alongside it.
- Growing organic in soil? BioBizz Top-Max. It's certified organic and sits in your normal watering.
- Bigger room, chasing weight? Rock Resinator, and the 5L, 10L and 20L sizes make it less painful per litre.
- Whichever you go for, read the overdoing it bit above first.
Need a base feed to go with it? Have a look at our hydroponic nutrients guide.
Quick answers
What is a bloom booster?
An additive you put in alongside your normal flowering feed. Most are PK boosters, extra phosphorus and potassium for when flowers are building. Others, like CANNA Boost or BioBizz Top-Max, work on the plant's metabolism instead.
When should I start using a PK booster?
It depends on the product. CANNA PK 13/14 goes in for 3 to 6 days about three weeks before harvest. Advanced Nutrients Big Bud runs in weeks 2 to 4. House & Garden Top Booster covers the final four weeks. Follow the bottle.
Can I use two bloom boosters together?
One PK booster plus one metabolic booster is fine. CANNA Boost, for example, is made to work with any PK. Two PK boosters at once is just doubling up and risks lockout.
Can too much PK booster harm my plants?
Yes. Your base feed already has phosphorus and potassium, and going well past what the plant can use can cause a nutrient lockout rather than bigger flowers. Stick to the dose and timing on the label.
Do bloom boosters work in soil and coco?
All six here work in soil, coco and hydro. BioBizz Top-Max is made for soil but is safe in coco and hydro too, and Advanced Nutrients make a separate COCO Big Bud for coco growers.
What's the best organic bloom booster?
BioBizz Top-Max. It's OMRI and SKAL certified organic and you add 1 to 3ml per litre at every watering from week 2 of flowering, stopping a week before harvest.
Still stuck? Give us a ring on 020 8686 9643 or email info@thehydrobros.com.
Or pop into the shop for a natter: Unit 12, Progress Business Park, Progress Way, Croydon CR0 4XD.
More grow guides
- Rock Resinator: a closer look
- Best CalMag supplements
- Best silica supplements
- Best root stimulators
- CANNA Coco A&B: a closer look
- Best pH and EC meters
The Hydro Bros Team. We sell this kit and talk growers through it every day in our Croydon shop.
