LED or HPS? Why the Best Growers Use Both

Tom Green founded Greens Horticulture in Bristol and has spent twenty years talking to growers across the counter. This is his view, from experience rather than a laboratory, on a question we get asked every week.

Most growers have switched to LED grow lights, and rightly so. A good LED bar fixture beats a 600 W HPS on yield per watt, runs cool, needs no lamp changes and does a superb job of vegetative growth and the first couple of weeks of flowering. If you are growing for quantity, or you are starting out, buy an LED and do not look back.

But if you are hunting for quality over quantity, there is something the growers who have been at this longest will tell you, a little quietly because it has become slightly taboo since everyone went all-LED: adding a high-intensity discharge lamp, an HPS, a metal halide or a ceramic metal halide, alongside the LED for the middle weeks of flowering noticeably improves the finished flower. Aroma, flavour, the density of the oils and resins, the way the crop finishes. In our experience it is not a small difference.

Think of it like a nutrient schedule

The way I explain it in the shop: the LED is your base feed, the NPK. It does the heavy lifting from cutting to harvest and you would never run without it. The HID is the additive, the bloom stimulator you bring in for a few weeks of flowering and then take out again. Nobody would feed a bloom booster in veg or run it as the only feed; nobody should run an HPS as the main light any more either. Run LED as the main light, add HID for the mid-to-late flowering weeks.

Why growers think it works

This is the part to read with the hedging switched on: it is the accepted grower explanation, not proven science. LED is cold light. It gives the plant photons and almost nothing else, and that is exactly why it is so efficient. Even LED fixtures with UV diodes give cold light. An HID lamp is a single, very hot point source: it gives radiant heat that lifts leaf temperature, a UV component, and (with metal halide and CMH) a broader spectrum, and it drives light down into the canopy differently from a flat panel spread across the tent roof.

Growers put the difference down to a small, deliberate amount of stress in weeks three to seven of flowering. A plant protects itself against UV and radiant heat by making more of its protective oils and resins, and those are the compounds that carry aroma and flavour. Under cold light that protective response is not triggered the same way. Whether that is the whole story I could not tell you; that the finished crop smells and tastes better is something our customers have been telling us for years, and it is what we see ourselves.

A suggested schedule

Vegetative growth: LED only, dimmed to what the plants need.
Flower weeks 1-2: LED only, at or near full power.
Flower weeks 3-7: LED plus HID, running together for the lights-on period.
Final week or two: grower's choice. Some take the HID out to cool the tent for the finish; some keep it in to the end.

Dimming: the LED makes room for the lamp

The typical UK set-up now is a 720 W LED bar fixture in a 1.2 m tent, and every one of ours dims: the Maxibright Varidrive 3.0 720 W from 25 to 100 per cent, the Adjusta-Watt Intec+ 720 W and the Gorilla 720 W likewise. Turn the LED down to around half and you have freed up roughly 360 W of power and heat budget for the HID. It is not a neat one-for-one swap, and plenty of LED growers never run at full power anyway, but it is the right way to think about it: you are moving part of the budget from cold light to hot light, not adding a second full light.

That is also why a dimmable HPS is ideal and a 400 W lamp is often enough. You are not after full HPS output; you are adding radiant heat and leaf temperature, not flooding the tent with light. A Varidrive ballast runs a 250, 400 or 600 W lamp and can be turned down (Maxibright advise against dimming metal halide lamps, so dim the HPS, not the MH). A 315 W CMH runs on its own ballast, not an HPS one, and gives the broadest spectrum of the three.

Placement: LED on top, lamp below

Bar-style LEDs can generally take a lamp above them, but in a tent it is usually better the other way round: move the LED to the very top of the tent and hang the HID below it, so the lamp's heat is not cooking the LED bars' heat sinks from behind. Keep the lamp a sensible distance off the canopy (45 to 60 cm for a 600 W, less for a 400 W) and check leaf temperature with the back of your hand or an infrared thermometer. You are not trying to raise the temperature of the whole tent much; you are warming the leaves under the lamp. A Cool-tube reflector (125 or 150 mm, lamps up to 600 W) takes most of the heat straight out through the ducting if the tent runs hot.

The extraction has to carry the extra load. Size it for the tent with the lamp in, not the LED alone: our fan and filter guide has the table, and going one fan size up is cheap insurance.

What you need

A dimmable ballast: the Maxibright Varidrive (250, 400, 600 W and a 660 W boost, soft start).
A lamp: 400 or 600 W HPS such as the Sunmaster 600 W Super HPS or an Osram Vialox NAV-T (400, 600 or 1000 W; Osram say change it every 8 to 10 months); a Maxibright 600 W in Dual Spectrum or Super Bloom; a Sunmaster metal halide if you want the blue end (Sunmaster and Powerplant both describe a halide as the lamp of choice for adding blue to a mostly-sodium room, at about one halide to two sodiums); or a Philips 315 W CMH on a CMH ballast.
A reflector: the Euro reflector (open, 250 to 600 W) or the Cool-tube. The Euro Grow Light bundles reflector, Varidrive and a 600 W Dual Spectrum lamp.
A timer or controller so both lights switch together: a plug-in Time-R or Optimum timer for one lamp, a Maxiswitch Quad contactor for several. Everything is under lighting controllers and accessories.
Extraction sized for the extra heat: see the fan and filter guide.

What it costs to run

Be honest with yourself about the electricity. An HID lamp uses more power per unit of light than an LED, which is the whole reason the industry moved on, and you are adding it on top of a light that already does the job. A 400 W lamp for twelve hours a day over five weeks is roughly 170 kWh; at current UK tariffs that is a few tens of pounds a cycle. For a grower chasing yield that is money better spent elsewhere. For a grower chasing the best flower they have ever finished, it is the cheapest upgrade in the room.

The short version

LED for veg and early flower, because nothing does it better. Add a dimmable HPS (or a metal halide or CMH) below the LED for weeks three to seven, with the LED turned down to make room for it and the fan sized for the heat. Take it out for the finish if you like. Treat it as the additive, not the base. And if you want to argue the point, come into the shop; it is our favourite conversation.

Browse HPS grow lights, LED grow lights and all grow lights. For choosing the LED itself, see LED grow light wattage by tent size.

More set-up guides

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.