Minimum PAR to grow plants

[VaughnH]

I have been growing some stems of Ludwigia Repens in my relatively low light tank, with high CO2 concentration. It grew pretty well, with full sized leaves, but much slower than usual. The last time I pruned and replanted, I only planted one stem of the Ludwigia, and put it back near the rear glass, very low in the tank - short stem. It did very poorly, eventually sprouting a couple of leaves, but finally rotting. So, figuring that was a low light area, today I measured the PAR intensity there - it was about 30 - 40 micromols. Where I had been growing it I tested the light and got about 40-50 micromols. This may indicate that the minimum light to grow Ludwigia Repens is 40-50 micromols.

Does anyone have similar data for other plants or even for this plant?

 

[SuperColey1]

No data but a couple of pictures to show the same for L Repens. Forgive the poor quality. these are old and was a poor camera:

This tank was 0.6WPG 1 x18W T8. 24" long on a 33USG 'tall' 30" tank, 15" from water surface to substrate.

Pic 1 is L Repens directly in the centre of the tank after a couple of weeks. This is obviously in the position where the light is peak. Super fast growth and spread wide rather than upward:

Pic 2 is the same plant moved to the rear and right behind the slate rock. Still grew well but took a month or more to reach the same size and upward rather than outward:

Small off note is that under that low level of light the plant remained a yellowy colour and only went peachy coloured at the tips. Not red as you see under 'high light' CO2 enriched tanks.

AC

 

[jeremyh]

I imagine that the stem in the back did not just have lower light - the spot it was tucked away probably had much lower CO2 levels too.

Bet that's what Tom would say, anyway.

 

[SuperColey1]

It was actually Nutrafin DIY CO2 at the time and the ladder was behind where the plant was move to so if anything it would have higher levels available.

AC

 

[VaughnH]

In my case, my CO2 comes up through the substrate, from a RFUG, all over the substrate area. It is possible that this area was low in CO2, since it is near the back glass, outside the RFUG grid. Given the low light level, the plant should not have needed much CO2, so I discounted that as the cause, but I'm not at all sure about that.

 

[Tom Barr]

The Minimum light that can grow plants is referred to as the Light compensation point(LCP).

This is a useful tool/concept is answering such questions.

I think most aquatics with CO2 enrichment and good nutrients sources(water column and/or sediment) have little issue going down to the 30 micromol ranges.

This plant grows all over, in forest, deep in 20ft of water on limestone, blackwater pools with a pH of 4.5, road side ditches........

Hydrilla has a LCP of 12 micromols(see reference from Bowes).

Like testing for algae, you will need to ensure CO2/nutrients are not an issue, current and other changes.

Then allow time for the plants to adapt. As you get closer to the LCP, growth slows down. So it becomes harder to keep the plant if you trim it often etc.
But ti should become easier if you want to slow the rates of growth down to reduce pruning.

That's a balance if the goals you have, but they can be influenced strongly by CO2/nutrients also.

So as you approach the bare min for light(or CO2 or nutrients), you start walking a razor's edge. So I think adding "just enough" can be done, but you will not get hardly any growth at all either. Better to leave a little buffer, then not worry so much and have such a fragile system that needs more tending.

The goal is reducing labor, not cutting it so close you bottom out.
Seems 30 is a good range.

Regards,
Tom Barr