Bloom Boosters and PK Explained: Shooting Powder, ROX and Timing

What is a PK booster, and when do you use it? A PK booster (also sold as a bud or bloom booster) is a concentrated phosphorus (P) and potassium (K) additive fed alongside base nutrients during flowering to support flower and fruit development. Classic PK 13/14 goes in as a short burst about three weeks before harvest; modern low-dose boosters like Buddhas Tree PK 9-18 run from around week 4 of bloom to the flush. Run one PK booster at a time, never two.

House & Garden Shooting Powder, ROX, Moonshine and the Buddhas Tree range all promise bigger yields but work differently and enter the schedule at different weeks. Here's what each is, when it runs, and the rules that stop them doing more harm than good.

Who this guide is for: UK home growers already running a base nutrient line who want to know which bloom additive to pick, when it runs, and how to avoid overfeeding.

What is PK 13/14, and how does it compare to modern bloom boosters?

CANNA PK 13/14 is a concentrated mixture of phosphorus and potassium added during flowering, when demand for both spikes. Phosphorus feeds flower metabolism and cell formation; potassium moves water, nutrients and sugars (the NPK basics guide covers what each element does). The "13/14" is its European labelling analysis (13% phosphorus as P₂O₅, 14% potassium as K₂O); the same bottle is NPK 0-10-11 on North American labels, which calculate differently.

The classic CANNA approach is a short burst: 15 ml per 10 litres (1.5 ml/L) roughly three weeks before expected harvest, in the tank for about 3–6 days. CANNA's own guidance warns that over-fertilisation harms substrate and plant.

Modern boosters go two ways: lower and longer, as with Buddhas Tree PK 9-18 at just 0.25–0.5 ml per litre from around week 4 of flowering to the flush, or away from mineral PK entirely. ROX and Moonshine work alongside your base feed rather than adding a slug of P and K.

Additive	What it is	Typical dose	Window (8-week bloom)
CANNA PK 13/14	Classic mineral PK shot (13% P₂O₅ / 14% K₂O)	15 ml per 10 L (1.5 ml/L)	3–6 days, about 3 weeks before harvest
House & Garden Shooting Powder	Powdered bud expander, 65 g (2.3 oz) sachets	1 sachet per 100 L, then 2	Final 3 weeks before harvest
Buddhas Tree PK 9-18	Low-dose liquid PK (9-18 analysis)	0.25–0.5 ml/L (hydro); 0.3–0.6 ml/L (soil)	Week 4 through to the flush
Buddhas Tree Flower Burst	Early-bloom PK (NPK 0-10-8) with boron	Follow the label	First weeks after the switch to 12/12
ROX Flower Enhancer	Additive: organic acids, humates, seaweed, PGRs, triacontanol	Follow the label	Weeks 4 and 5
Moonshine Nutrient Enhancer	EC-neutral biostimulant (molasses, kelp, botanical oils)	1 ml/L	All stages until the flush

None of these replaces your base feed. They assume pH and EC are already dialled in (our pH and EC guide).

When should each bloom booster enter and exit the schedule?

PK 13/14 goes into the tank about three weeks before your expected harvest date and stays for roughly 3–6 days: week 6 of a standard 8-week flowering cycle. The key rule with any classic PK is to count backwards from harvest, not forwards from the light flip.

Across a reference 8-week bloom, this is not a combined schedule: each row shows where that product sits if it's the one PK booster you run.

Additive	Wk 1	Wk 2	Wk 3	Wk 4	Wk 5	Wk 6	Wk 7	Wk 8	Flush
Buddhas Tree Flower Burst*	✓	✓	✓	—	—	—	—	—	—
Buddhas Tree PK 9-18	—	—	—	✓	✓	✓	✓†	✓†	stop
ROX Flower Enhancer	—	—	—	✓	✓	—	—	—	—
CANNA PK 13/14	—	—	—	—	—	✓ (3–6 days)	—	—	—
H&G Shooting Powder	—	—	—	—	—	✓ 1 sachet/100 L	✓ 2 sachets/100 L	✓ 2 sachets/100 L	stop
Moonshine	✓	✓	✓	✓	✓	✓	✓	✓	stop

* Indicative: Flower Burst targets the spurt after the 12/12 switch; follow the label for the exit week. † Doubled dose (0.5 ml/L hydro / 0.6 ml/L soil) for the last two weeks before the flush only.

Everything marked "stop" ends at the flush, when you switch to plain pH-adjusted water or a flushing solution such as CANNA Flush (how to flush properly).

Longer-flowering varieties shift everything: anchor the finishers to your harvest date and stretch the middle weeks. CO₂ enrichment also changes how hard plants work in this window (our CO₂ enrichment guide).

How do Shooting Powder, ROX and Moonshine differ?

House & Garden Shooting Powder is the heavyweight finisher: a powdered bud expander in 65 g (2.3 oz) sachets, claimed to trigger a fresh flowering cycle after the normal one stops, with output increases of up to 20%. It's the strictest product here: healthy plants only, never alongside another PK booster, and base nutrient down to EC 1.2 while it's in the tank:

Timing	Dose	Base nutrient
3 weeks before harvest	1 sachet/100 L	Adjust to EC 1.2
2 weeks before harvest	2 sachets/100 L	Adjust to EC 1.2
Final week	2 sachets/100 L	Adjust to EC 1.2

Top Shooter is House & Garden's liquid version of the same concept: run one or the other, never both.

ROX Flower Enhancer is a bloom additive built on organic acids, humates, seaweed extract, plant growth hormones (PGRs) and triacontanol, run alongside your normal feed during weeks 4 and 5 of an 8-week cycle: mid-bloom, not a late finisher. Published doses vary between listings, so follow the label on your bottle. Some growers prefer to avoid PGR-based enhancers, so check the ingredients and decide.

Moonshine Nutrient Enhancer isn't a PK booster at all: a biostimulant of molasses, kelp and botanical oils, run at 1 ml per litre through every stage until the flush (0.5 ml/L as a foliar spray; Moonshine also makes a ready-to-use Sunshine Foliar). It isn't a mineral PK product, and per the maker it's EC-neutral and doesn't shift pH, so it can sit alongside a PK booster where a second mineral product can't.

CANNA Boost Accelerator is another mid-bloom option, but a companion rather than a rival. CANNA describes it as an additive, not a nutrient, that increases the plant's photosynthesis, and it recommends pairing it with PK 13/14. Treat it as part of the feed programme, not as a second PK booster, and follow the label for dose (Boost Accelerator).

How much PK booster is too much? EC ceilings and overdriving

Every millilitre of mineral PK you add is dissolved salt, and dissolved salt is EC. The classic mistake is adding a booster on top of a full-strength base feed. The result is leaf-tip burn, lockout and a stressed finish (symptom-spotting: our deficiency and flushing guide).

Your ceiling is set by your growing medium: the total EC of base feed plus booster has to stay inside the range your medium tolerates in flower, so a booster eats into headroom the base feed is already using.

Medium (flowering)	Ceiling for base feed plus booster (mS/cm)	Watch-out
Recirculating hydro	~1.2–2.2	CANNA: max 1.3–1.7 above your source-water EC
Coco coir	~1.1–2.0	Buffers less than soil, so check runoff EC closely
Soil	~1.4–1.7	Lowest ceiling; least forgiving of a PK spike

These are ceilings, not feed targets. Stage-by-stage EC targets live in the pH and EC guide and the full-cycle feeding schedule.

The manufacturers tell you how to avoid overdriving:

• Make room for the booster. House & Garden's EC 1.2 base-nutrient rule while Shooting Powder is in the tank says it plainly: the booster replaces base-feed strength, it doesn't add to it.
• Keep the window short. CANNA's PK 13/14 window is 3–6 days, with an explicit over-fertilisation warning. Longer is not better.
• Measure, don't guess. Mix the base feed, add the booster, then check with an EC meter such as the Bluelab Truncheon Nutrient Meter, and hold total EC where your feed chart puts it.

Low-dose designs make this easier: at 0.25 ml per litre, Buddhas Tree PK 9-18 adds far less salt per feed than a classic 1.5 ml/L shot, which is why it can run for roughly five weeks while PK 13/14 lasts days. In soil, feed it only every second watering.

Can you stack two PK boosters together?

No. Two PK boosters at once double up phosphorus and potassium without doubling the benefit, and push EC past sensible limits. House & Garden state it outright, "do not use Shooting Powder in combination with any other PK booster", and Buddhas Tree PK 9-18 guidance carries the same one-at-a-time rule.

What can run alongside a PK booster: EC-neutral biostimulants like Moonshine, companion additives like CANNA Boost Accelerator, root-zone additives (enzymes, microbes, silicon; see our root-zone guide), and Cal-Mag such as CANNA Cal-Mag Agent where your water calls for it. Sequential use isn't stacking: Flower Burst exiting before a mid-bloom PK enters is a schedule, not a stack. Keep one mineral PK in the tank at a time, and follow the brand's feed chart where one exists.

FAQ

When do you use PK 13/14? Add PK 13/14 about three weeks before expected harvest and keep it in the tank for 3–6 days at 15 ml per 10 litres, per CANNA: week 6 of a standard 8-week cycle. Count backwards from harvest, not forwards from the light flip.

Can you use two PK boosters at the same time? No. Two PK boosters double up phosphorus and potassium and push EC past sensible limits. House & Garden say not to combine Shooting Powder with any other PK booster. Run one mineral PK at a time; EC-neutral biostimulants like Moonshine can sit alongside it.

What is House & Garden Shooting Powder? Shooting Powder is a powdered bloom booster ("bud expander") from Dutch manufacturer House & Garden, sold in 65 g sachets dosed per 100 litres during the final three weeks before harvest. It's a horticultural additive, no relation to firearms powder. House & Garden claim output increases of up to 20% and advise healthy plants only.

Do bloom boosters actually work? They can, as a finishing step on an otherwise healthy grow. A PK booster supplies phosphorus and potassium when flowering demand peaks, and helps most when base feed, pH and EC are already right. Yield claims such as House & Garden's up to 20% or Buddhas Tree's up to 40% are the makers' figures, not guarantees, and no booster fixes poor light or watering.

Recommended kit and further reading

The boosters in this guide, from The Grow Superstore:

• House & Garden Shooting Powder: sachet finisher for the final three weeks; remember the EC 1.2 base rule.
• CANNA PK 13/14: the classic short-burst mineral PK, also in the Canna Nutrients & Soils collection.
• ROX Flower Enhancer: the two-week additive for weeks 4 and 5.
• Moonshine Nutrient Enhancer and Moonshine Sunshine Foliar: the EC-neutral biostimulant for every stage, plus the ready-to-use foliar.
• Buddhas Tree PK 9-18 and Buddhas Tree Flower Burst: the modern low-dose pairing; browse the Buddhas Tree collection.
• Bluelab Truncheon Nutrient Meter: check tank EC every time a booster goes in.

Most relevant collection: if you only click one link, start with Additives, where the boosters in this guide are grouped.

Related guides: CO₂ enrichment in bloom · pH, EC and water quality · root-zone health · deficiencies, lockout and flushing · the full-cycle feeding schedule.

Why trust The Grow Superstore

We've been advising UK indoor growers since 2009, online and from our Peterborough store at 6 Wulfric Square, Bretton, PE3 8RF. The advice we give at the counter every week is the same: pick one PK product, time it from your harvest date, and make room for it in your EC rather than piling it on top. Pop in for a free in-store consultation and we'll match a booster to your base line, or order online with free delivery over a set order value on UK Mainland orders (exclusions apply).