Natural PK Bloom Hardener – Grenade Black

The bottle costs £40. The feed costs pennies a litre.

Most people judge a bloom booster by the price on the bottle. That's a bit like judging a car by the price of its key. Grenade Black is so concentrated that even at the full recommended dose of 0.4ml per litre, a single 100ml bottle treats 250 litres of feed – about 16p per litre of water. Many PK boosters ask for 1–2ml per litre. The grenade isn't the expensive bit. It's a reminder that small things can carry a lot of force.

What it is

Grenade Black is an advanced hybrid organo-mineral PK+ flowering additive, blended as a natural alternative to synthetic chemical hardeners. It doesn't force a plant into shape – it gives it what it needs to reach its own genetic potential. Phosphorus and potassium drive flower and fruit set, while amino acids, minerals and biological cofactors support plant energy levels (ATP) and the production of secondary metabolites: the compounds behind aroma, flavour and colour.

Why growers rate it

• Denser, heavier, healthier flowers and fruit through the critical mid-to-late bloom window
• 100% natural PK+ hardener – no synthetic growth regulators
• Ultra-concentrated – 100ml treats 250–500L, 1L treats 2,500–5,000L
• Plays well with others – compatible with all nutrient regimes in hydro, coco and soil

Not just for the grow tent

Anything that flowers or fruits can use the help: tomatoes, chillies and sweet peppers, strawberries, aubergines, cucumbers and flowering houseplants. Whether you're growing in a greenhouse, a conservatory or on a sunny windowsill, this is the late-season feed that turns flowers into a crop.

How to use

• Dose: 0.4ml per litre for full effect (0.2ml per litre is the minimum). Don't go above 0.4ml – more isn't better past that point.
• When: weeks 3 to 7 of the flowering cycle. Start in week 3 and use it at every feed through to the end of week 7, then stop.
• Continuous croppers (tomatoes, chillies, peppers, strawberries): start once the first fruits have set and use it at every feed through the main fruiting period.
• Order: add Grenade Black to your water first, then top up with base nutrients and other additives to reach your target EC.
• Storage: keep in a cool, dry place.

🌱 Rose's Pro-Tip

Don't ration it – there's no prize for a half-full bottle at the end of the season. At full dose a 10 litre watering can needs just 4ml, so use the dropper or a small syringe rather than eyeballing it. And let Grenade Black be your only PK booster while you're using it – stacking two PK products at once is how plants get overloaded.

Better together: Grenade White

Grenade White builds the frame from seed onwards with stabilised silicic acid; Grenade Black fills it out in bloom.

Free UK delivery on every Grenade order – no minimum spend.

Grenade Nutrients, stocked by Black Flag Horticulture, Coventry. The 5 litre size is made to order, so please allow a little extra time for dispatch.