Best Spectrum For Hardwood Cuttings

[ShadyStump]

I've tried several searches, but nothing helpful is coming up.
What's the best section of spectrum for rooting hardwood cuttings?

I have a hodge podge collection of grow lights, so many options for what to use for my cercocarpus ledifolia cuttings I stuck the other day, but I'm unsure what would be best. Full spectrum, red/blue? I have one with a setting for seed starting that's primarily blue.
They're in a large north facing window, so some supplemental lighting is needed, and I can't find a heating mat that will fit both my needs and my budget. So I'm hoping that providing the best lighting I can will help increase my success.

Thanks for any help!

 

[Wires_Guy_wires]

I'm a big fan of cool white. We used those in the labs, and I use them in my terrarium. More is better of course, and I think wavelength is just one part.. The amount of effective light matters as much.

Have you checked heating cables for reptiles? Make your own mat ;-)
Mine doesn't exceed 45 degrees C so it's safe in a woodchip bedding.

 

[Glaucus]

Full spectrum is always best. You cannot even buy (good) grow lights that are not full spectrum. Nowadays, all LEDs are full spectrum. HIDs are full spectrum, MH are.

A spectrum is either heavy on blue or heavy on red. If it is heavy on red, the light is more efficient. And this can lead to more elongated growth. Light with more blue intensity will be less efficient, but growth will be more compact.
For LEDs, this is achieved by either having only a mix of cool and warm whites, plus some reds (reddish, elongated growth). Or just a mix of cool and warm whites (blueish, more compact growth).
For rooting a hardwood, this debate is entirely irrelevant.

For rooting, you only need a little bit of light. For good growth after rooting, that is another debate. Humidity, bottom heat and possibly a fungicide are the thing that determine success for (hardwood) rooting itself.
A heating mat will be cheaper than a good light. I got the impression you bought a hodge podge of bad grow lights, and now you are out of money but you don't have what you need.

 

[ShadyStump]

I'm a big fan of cool white. We used those in the labs, and I use them in my terrarium. More is better of course, and I think wavelength is just one part.. The amount of effective light matters as much.

Have you checked heating cables for reptiles? Make your own mat ;-)
Mine doesn't exceed 45 degrees C so it's safe in a woodchip bedding.

I've not looked into reptile heaters. That's a great idea I'm sure I've heard elsewhere. Not sure how I forgot. Thanks!

So I'm hearing that from your experiences the light is of little consequence until it leafs out, long as there's SOMETHING for it?

Edit: to clarify, I'm no stranger to cuttings, just never hardwood cuttings like this. I don't know where the separations and similarities are yet.

 

[Glaucus]

I've not looked into reptile heaters. That's a great idea I'm sure I've heard elsewhere. Not sure how I forgot. Thanks!

So I'm hearing that from your experiences the light is of little consequence until it leafs out, long as there's SOMETHING for it?

Edit: to clarify, I'm no stranger to cuttings, just never hardwood cuttings like this. I don't know where the separations and similarities are yet.

Depends. If there are no leaves at all because it is a deciduous hardwood, there is very little photosynthesis (some species have some chloroplasts in their stems). So then it is even less important.

But if he hardwood has leaves, the light will lead to photosynthesis in the leaves and generate more sugars. Which should help the roots grow faster. With an emphasis on 'should'. It may also just cause the leafy parts to grow independently of any rooting. In the end, rooting is about humidity loss vs the time it takes for the cutting to grow roots that can support the leaves it has. Bottom heat will speed it up.

I would say that maybe the grow lights are most useful overwintering the cutting once it has rooted. If something roots by December, you don't want to move it outside into a zone 6 winter right away.

 

[ShadyStump]

Depends. If there are no leaves at all because it is a deciduous hardwood, there is very little photosynthesis (some species have some chloroplasts in their stems). So then it is even less important.

But if he hardwood has leaves, the light will lead to photosynthesis in the leaves and generate more sugars. Which should help the roots grow faster. With an emphasis on 'should'. It may also just cause the leafy parts to grow independently of any rooting. In the end, rooting is about humidity loss vs the time it takes for the cutting to grow roots that can support the leaves it has. Bottom heat will speed it up.

I would say that maybe the grow lights are most useful overwintering the cutting once it has rooted. If something roots by December, you don't want to move it outside into a zone 6 winter right away.

No leaves here. I held them over in my fridge since October until I could get them started. I'm hoping I have my timing- and success rate- right to have live trees to move outside once the frost danger is over. Usually around May here.
They're curl leaf mountain mahogany, a semi-arid deciduous species that never quite loses it's foliage in winter, so I've been treating the air layer I separated in the fall essentially like a pine- dormant but still needs plenty of light.
That was my concern with these cuttings; how much light do they need even without foliage?

 

[Wires_Guy_wires]

Auxins will break down faster under the influence of light and heat.
For the most common: indole acetic acid (IAA) the highest excitation is in the UV sprectrum. But as we see with strong blue and full spectrum lights, growth gets more compact. There's either less auxin production or a higher destruction of it.

You'd want the brightest lights you can get, because when they take root.. You'll want to put them outdoors eventually. Cuttings can have weaker roots and if they can't build up some strength and light resistance, they'll wilt as soon as they go outdoors. I've had that happen with some cherries and plums.