Question 1

Do grow lights work, and does the colour matter?

Accepted Answer

They work, and the colour matters less than the **intensity and duration** — which is the reverse of what most marketing implies.

**What plants actually use.** Photosynthesis is driven mainly by light in the **red** (around 600–700nm) and **blue** (around 400–500nm) parts of the spectrum, because that is where chlorophyll absorbs most strongly. Green light is reflected rather than absorbed, which is why leaves look green.

This is where the purple grow lights came from: supply only red and blue, waste nothing on green.

**Why that reasoning is incomplete.** Green light is not useless. It **penetrates deeper into the leaf and into the canopy**, reaching cells that red and blue never get to, and studies have found it contributes meaningfully to photosynthesis in real plants rather than isolated chlorophyll. Full-spectrum white LEDs have largely displaced purple ones in serious use — partly for this reason, and partly because you can actually **see** your plants and spot problems.

**What the different wavelengths influence beyond raw energy:**

**Blue** tends to produce compact, sturdy growth with thicker leaves. Insufficient blue is associated with stretching.

**Red** drives stem extension and, with far-red, flowering responses.

**Far-red** affects the shade-avoidance response — plants detect it and stretch, believing they are being shaded.

**UV**, in small amounts, can increase production of protective compounds.

**The two things that actually determine results:**

**Intensity, measured properly.** Lumens measure brightness **to the human eye** and are close to meaningless for plants. The relevant measure is **PPFD** — photosynthetic photon flux density — and how it falls off with distance, which it does steeply.

**Duration.** Total daily light integral matters more than peak intensity. Most houseplants do well with 12–16 hours; many flowering plants respond to **day length**, so more is not always better.

**A bright windowsill delivers far more light than people estimate**, and a cheap low-wattage lamp far less.

Question 2

Why do plants flower when they do?

Accepted Answer

Mostly because of day length, which is the only environmental signal reliable enough to time a life cycle by — temperature varies wildly from year to year, but the length of the night on a given date

Question 3

Why do leaves change colour in autumn?

Accepted Answer

Because the tree withdraws its most valuable resources before discarding the leaves — and the colours reveal what was there all along, plus something newly made. The green is chlorophyll, the pigment

Question 4

Why has my variegated plant turned all green?

Accepted Answer

It has reverted — produced growth without the variegation — and if it is not removed it will take over the plant entirely, because the green growth has a genuine advantage. Why variegation exists.

Question 5

Why do plants grow towards the light unevenly?

Accepted Answer

Because growth is controlled by a hormone that redistributes to the shaded side, making that side elongate faster — so the plant bends without any part of it pulling or pushing. The mechanism. The

Question 6

Why did my hydrangea change colour?

Accepted Answer

Because the colour of a mophead or lacecap hydrangea is set by soil chemistry rather than by the variety alone, and something about your soil has shifted. The mechanism is aluminium. Blue pigment

Question 7

What can you actually grow in a shady garden?

Accepted Answer

A great deal — provided you first establish what kind of shade you have, because the word covers conditions that are completely different from a plant's point of view. The distinctions that matter:

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