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Full spectrum or blurple for houseplants

Blurple lamps (red and blue LEDs, little else) grow plants. They also paint the room purple. In a flat, that second fact usually decides the purchase.

What the plant is using

Leaves use red and blue light heavily in photosynthesis, and they also use green. A lamp that emits only red and blue can grow a plant and still look like a nightclub. A white full-spectrum LED includes those wavelengths and the rest of the visible range, so the room still looks like a room.

The RHS artificial lighting guide says plants absorb mostly blue and red, that blue tends to drive leafy growth, and that red matters for growth and flowering. It also says that too much blue and almost no red can leave plants stunted, with dark leaves, thick stems and few flowers. That is an argument for a mix, not for a purple glow in the sitting room. A white horticultural LED is one way to supply that mix without the colour cast.

Colour temperature, in kelvin, describes how white looks to you. The SANSI bulbs in the shortlist are stated at 4000 K, a neutral white. The Barrina magnetic 2 ft pack is stated at 5000 K, which is a cooler daylight white. Neither is a warm 2700 K reading-lamp yellow, and neither is blurple. "Warm white" in ordinary lighting often means 2700–3000 K. A 4000 K grow bulb is usually the one people can live with, because it is white and still bright enough to read by.

When blurple is a reasonable choice

A spare room, a closed cabinet you do not sit beside, or a seedling shelf in a garage can use a red-and-blue lamp without annoying anyone. The plants do not need the light to flatter the wallpaper. If that shelf is visible from the sofa, choose white.

Blurple is also a clue that the listing is aimed at a tent. If the rest of the page is extractors, carbon filters and "bloom boost", you have left houseplant territory. High-output tent lights are Hydroponics Guide territory.

Words that do not mean "good spectrum"

"Full spectrum" on a box is not a standard you can enforce. It should mean the lamp covers the visible range rather than two spikes of red and blue. It does not mean the spectrum has been measured in your room, and it does not mean the lamp is bright.

Kelvin without a wattage is incomplete. A 4000 K bulb at 5 W and a 4000 K bulb at 40 W are not the same tool. Watts, distance and the "equivalent watt" trap are in what listings exaggerate and PPFD and distance.

For a living room, a kitchen, or a shelf you walk past: white full spectrum, on a timer, at the maker's distance. For a hidden seedling shelf: white or blurple, as long as the watts and the distance are honest. The format around that choice is in bulb, bar, panel or gooseneck.