by Rob Smythe MSc
Question #1: To be asked what is the best level of shade cloth to grow neoregelias under does not sound too difficult a question.
Answer: In the Townsville region in my opinion, the best neoregelias, colour wise, are grown under 50% shade cloth. These are grown by Pat Coutts under a moderate fertilizer deficit. They are fertilized only once, and that is during potting.
Question #2: What colour mesh? This was the next question. I am not sure of the answer to this as I do not use mesh but use my trees for shade. I will suggest some things to consider.
Warning
If you like to think things through, like mind teasers, like answering questions with more questions and don’t mind a bit of fuzzy science—read on.
If you are bored stiff with such interests skip directly to ‘Outcomes’.
Something about shade for bromeliads that is better than a guess.
I will come back to question one in detail later on. In answer to question 2.
I can see light coming to plants in a bush house by two different mechanisms.
This raises the question. Is there any advantage using one colour over another?
White-mesh obviously transmits a balance of colours and absorbs some of the heat and UV radiation. This would be most suitable where low shade is required.
Green-mesh: It absorbs the photo synthetically useful light and transmits the green light which is of no use to the plant. This would be useful where you are over loading the plant with useful light. See discussion further on. I have just given you the reason why plants appear green, they throw green light back at you as they can’t use it. It is useful to you though as it helps you see your way around in otherwise high shade.
Other Colours: So if on the other hand you wanted shade cloth removing useless radiation such as UV, Green light and heat while transmitting the most useful light for your plant you would want orange/red or blue or even a mixture of the two as these light frequencies are optimal for absorption by chlorophyll-a and hence photosynthesis. I think I would give orange/red and blue a miss and go for ‘off white’. This off-white mixture of the above light colours, would look OK. Sorry, not the place to argue that orange/red and blue light of similar intensities when mixed actually appears whitish rather than blackish as you would find using your paint box colours. Take the word of a person who majored in chemistry and physics and specialized in Atomic Absorption Spectroscopy and still had to look up the World Book Encyclopaedia for this answer. If you are stubborn like me and don’t want to be brushed aside just slowly turn your green gun down on your TV while looking at a white image (kidding but it would prove a point though not improving your popularity with the family).
Answer to question one continued.
Now getting back to the first question, I ask my self, is there an objective answer rather than my subjective answer? I warn you before I get too deeply into this, where I fail to achieve the perfect answer, that maybe you should take the above answer as gospel. The first thing I looked into was my own garden. All plants grow in the same soil and watered on sprinkler days. Some very large plants are grown in virtually empty pots once they are big enough to go into 10 inch pots. There is probably enough fertilizer in the soil for optimum growth. They occasionally would suffer from water stress. Remember I am in the Dry Tropics. The ones in empty pots would have some fertilizer stress. Any fertilizer they get is from the wells. The pots mainly contain foam to stabilize the plants
I have carried out some experimental work looking at the best growing and the best colouring regions in my garden. I looked around my garden and found firstly where the Neos coloured up best. Here I had the following Neos. growing, ‘Coconut Ice’ ‘Ferny Grove’, ‘Red Veil’, ‘Paula’, ‘Enchantment’, ‘Orange Sun’, ‘Georges Prince’, ‘Serendipity Girl’, ‘Flesh’, ‘Blood Plum’, ‘Red Pride’ and some of my own ‘Roy’s Rogue’ hybrids. I took light intensity (flux) readings every half hour as the sun passed overhead. The highest reading of light impinging on Neo Serendipity Girl was recorded. The collected data is shown in figure #1.
This data was collected every half hour over a period from 7.30 am to 5.30 pm.
The shade averages out at 60 % which suggests 50% shade cloth would be better than 75%. As this data stands this second answer is in agreement with my original, subjective, answer.
Most vigorous plants.
Over several years, a large selection of my plants, have been conditioned to many hours of full tropical sun. I chose for my experiment what I believed were the biggest, healthiest (best grown) if not necessarily the most coloured plants in the garden. Plants in this, best grown, group are the following Neoregelias: ‘Gee Whiz’, Princess Grace x Gee Whiz, ‘Barbarian’, ‘Red Gold’, ‘Morrisoniana’, ‘Georges Prince’, ‘Samoan Chief’, cruenta, and to my knowledge, an as yet unnamed plant called N. Brazil which was made famous in a photo with Grace Goode’s hand appearing so small in comparison to the plant. These are growing in the ground. These are separated by a pathway from the ones receiving maximum sun so are still in a high light area. The higher light side gets full tropical summer sun from 10 am to 3.30 pm. The (best grown) group has only one hour of full sun due to receiving shade from a palm tree and a tree fern. I have monitored the light intensity (flux) in detail, see figure #2. I measured the light falling on to one plant N. ‘Morrisoniana’ (I believe that name stands, some call it N. Rosy Morn), a well coloured up plant growing amongst this (best grown) group. As this plant was fairly large I got some variation of reading over the plant. I recorded the highest value. The data for this and the previous experiment was recorded half hourly over three days from Dec 4th to 6th 2005 (Dec 10th the sun is directly overhead in Townsville). The data is presented above and coincidentally the average shade was also found to be 60%. The sudden drops in light intensities are due to the wood of tree branches coming between the plant and the sun.
Again this value of 60 % might suggest that 50% rather than 75% shade cloth would be the better choice. So far the three answers are all the same.
Unfortunately the above calculations are flawed. You can not average shade values in this way. Plants don’t see it as simple linear mathematics. They can only use so much flux of sunlight and the remainder is wasted. From here on the science gets a bit messy. To be precise I would need the facilities of a very advanced lab not just my little pocket size fluxmeter in order to scratch out an exact answer. If you just wanted to grow Till pueblensis in your bush house, later I will give you a reasonable answer but extrapolating research done on this plant to neoregelias might be a little tentative.
The comment above about not being able to average shade levels comes from work by Benzing and Renfrow1. They found that with Till pueblensis once light levels impinging on a leaf reached around 5300 foot candles (F-C), higher levels of light did not give higher levels of photosynthesis. So by way of an example let me explain why averaging data as I collected is not really valid. Averaging, we would say, two hours of light at 5300 foot candles is the same as one hour at 10,600 ft candles plus one hour in the dark. This is not at all how the plant sees it. The metabolic outcome is very different. The first produces twice the amount of sugar as in the second case. The light intensity in case two is twice what the above plant can cope with so it only utilizes half of it. Take my word that there is even a second reason that averaging does not work. I warn you that this is a mental teaser but I will spare you reason number 2 Every time I dig up new information I find two more holes to fill. I used findings of the above authors 1 to create this example. To reiterate their findings, for T.pueblensis experiencing above 5300 Foot Candle Power of light, the extra light has no further use in photosynthesis. Today I was lucky enough to find a plant, in Townsville, called T.pueblensis. I have also found literature stating T. fasciculata reflects around 50 % of incident light. I did a quick trip around my limited supply of tillandsias and got a reading up to 66% reflection for T.usenoides and around 10 to 15 % for my neoregelias. T. pueblensis gave a reflectance of 10%. I should not have been so surprised as this is a greenish plant from high altitude. This data I thought would give me the connection I wanted so that I could extrapolate existing data 1 known for Tills. and apply it to Neos. but low and behold I dug yet another hole.
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I thought in my travels to going nowhere, that I had the desired cross over point. Unfortunately it is another case of comparing apples and oranges. The trichomes of grey tills. are reflecting primarily white light whereas my neoregelias are reflecting the unwanted green light and T.pueblensis would be somewhere in between. My flux meter is at it’s most sensitive in the green area while in the region where chlorophyll-a functions best the meter is virtually useless. So really I can’t compare white light reflection quantitatively with green light reflection. That is why earlier I said I would need access to a very sophisticated laboratory. Fortunately the readings are fairly low at 10% reflectance for Neos and T.pueblensis. To move on I will assume the reflected light is all unwanted green light.
So far I have not been able to answer question #1 absolutely accurately but we must be getting close. We will explore the data that I have recorded as there is better to come.
The diagram (Figure #2) shows my own recorded data from the previously described experiment using my ‘best grown’ group of plants. The blue columns are my calculated and optimal levels of light for neoregelia growth (57,000 lux, 5,300 FC) see 1. This figure could be up to 10% too high. The red columns represent the light flux impinging on my plant of N. Morrisoniana, and is recorded every half hour throughout the day. I think for the fourth time I can predict (but now with some accuracy) that optimal level of light would be achieved with 50% shade cloth. This is obvious from figure two where you can see that maximum light (no shade) is twice the predicted optimum light level.
I rest my case on recommending 50% shade cloth for growing Neoregelias.
Other interesting questions.
Figure #2 shows 8 half hour blocks equalling or exceeding the maximum useful range and probably another totalled 5 to 6 half hour block equivalent for the bits and pieces that are lower than the optimum flux. This means that my own personal best growing area gets the equivalent of about 7 hours of a day of optimum or higher light. Doesn’t answer any of the questions put to me but does precipitate another couple of unanswered questions.
The research 1 and my data concentrates on the exposed leaf surface. How much do the self-shaded lower leaf surfaces contribute to overall photosynthesis? Measuring one plant Neo Champers I found about 10% of light was reflected and about 80% absorbed by the leaf with 10 % transmitted. Going through the next leaf down 25% was transmitted. This tells me three things.
1) There could be quite significant photosynthesis going on in the first shaded leaf.
2) As % transmission is increasing as we move towards base of the plant we may be seeing only photo synthetically useless light (green) being involved.
3) We are not comparing apples with oranges as much as I thought. A thick sparsely leaved non-transparent Till. Plant equates nicely with a transparent multi layered Neoregelia
Further questions arising.
T. pueblensis is a C3 plant which means its pores (stomata) are open and processing light and carbon dioxide during daylight hours. Closed at night. Fairly easy to monitor in the lab. Neoregelias are CAM plants and they open their pores at night absorbing and converting carbon dioxide. Pores close in the morning and concurrently light is needed to reconvert stored chemicals back to carbon dioxide which then undergoes photosynthesis to form sugars and other organics via the same Calvin-Benson cycle as found with the C3 mechanism.
New questions arise, none of which I can answer using the above data.
Q1) What limits the photo synthesis to 5300 F-C in the mentioned Tillandsia? Was it set by the rate of diffusion of carbon dioxide into the plant or a rate limiting chemical step in the process leading to sugar?
Q2) Since with the Neoregelia carbon dioxide products are all locked inside the vacuole of the leaf during the night, how many hours of light at optimum level are needed to process the stored nightly supply? My best grown plants suggest that this figure would be 7 hours or less. Research that I uncovered subsequent to my experimentation, confirms this 2. This new data suggests to me that for a CAM plant (non bromeliad) that after a night temperature of 15C the plant starts to uptake carbon dioxide again after 8 hours of light dropping to about 5 hours with a night temperature of 36 C. Townsville’s night temperatures are hovering around the mid twenties and their data 2 suggests about 6 to 7 hours of light would be enough.
Q3) Is the Light Saturation Point (the level of light above which photosynthesis stops increasing) the same for both types of plants (C3,CAM)? I would expect not, as CAM plants have the extra step of needing light to break down the night’s storage of malic acid and other stored organics.
Q4) Who has the time, the money and the qualifications and youth to do a PhD on Light Saturation in Bromeliads?
OUTCOMES
Shade cloth
In summary all the above says is that if you need to reduce very bright light, say 90% shade, use green mesh unless you can see well in the dark. If you are struggling for enough light (eg need 30% can only buy 50%) go for one of the off-white colours.
Optimum light requirements/ shade for some bromeliads
Today is a bright clear summers day in the tropics with a full light intensity of 12,500 FC.
If you are growing your plants under light limiting conditions i.e. adequate fertilizer, moisture available and no heat stress my findings and calculations suggest you will get optimum growth as follows:
A) Till pueblensis needs a constant 5300 FC 1 of light all day so shade should be
60 %. Cloth available 50%.
B) Guzmania lingulate needs 1500 FC 1 of light so shade should be 88% maximum. I say maximum as I have not factored in utilization of lower leaves for photosynthesis. G. lingulate is a C3 plant but some guzmanias are CAM plants, so my suggestion has conditions if you extend it to other guzmanias.
C) Neoregelias as for Till pueblensis with one other consideration. The Till is basically a green plant while the Neoregelia is brightly coloured. Extra light or extra stress or both would be required for optimum colour with Neoregelias.
My learned opinion after all this is: To grow Neoregelias to their best use no more than 50% shade cloth with fertilizer stress and at least 6 to 7 hours per day of optimum or higher light intensities. Temperature? I would like to keep my plants under 35C but don’t think that will be possible in the tropics. We have had one day at 41C without obvious damage. I have measured temperatures in the hottest area of my garden adjoining paving. I was amazed that water temperatures in the wells were as high as 43C, leaf temperatures were up to 39C, ambient temp 33C. These of course are very hardened plants but not bleached. How do they as CAM plants with all pores closed during the day survive this heat? Maybe this is why they carry a pool of water. Protect from drying winds and keep wells topped up are other condition for growing neoregelias well. A special recommendation for the dry tropics. CAM plants need cold nights to absorb carbon dioxide and hence to photosynthesise. We don’t have these conditions in summer so keep the plants well watered especially in the evenings. Stomata are known to open up with cooling. They will open at times, going against the rules in CAM plants, if there is no water stress 3.
The amazing bromeliads
Bromeliads are amazingly adaptive. I have areas where the shade is 99% in my garden. Grey leaf tills have germinated in there and they are an iridescent green and looking extremely healthy. I have ferns and guzmanias also growing in this very low light area. The guzmanias look to be struggling. They survive but don’t flower. Now to the other extreme, I have quite a lot in Neoregelias growing in full tropical sun during the hottest times of the day. It has taken suitable plants (about 20 concentrica/cruenta types) about 2 years to adapt. N. ‘Beetroot’, N. ‘A Perfect Score’, N. ‘Tossed Salad’, Neo ‘Gee Whiz’, Neo. ‘Isabel’ (sport), Neo. ‘Bruiser’, Neo ‘Hagar’ and various N. concentrica species and all are actually thriving. N. ‘Stars and Bars’ is not as attractive due to yellow carotene colouring but I will give it another year. Some plants, N. ‘Rosatina’, N. ‘Princess Di’, N. ‘Princess Grace’, N. ‘Lilac Dream’, N. Rio Grande(name not be registered), N. ‘Heart Music’ and N.‘Bruiser’ started colouring but faded. They are now the carotenoid green/yellow. It is early December and day temperatures are around 33 C, full tropical sun reaches these plants around 10 am and drops off sharply after 4 pm. That is 6 hours and many of these plants have taken only two years to adjust. Many bleached the first summer, none yet this summer. I must admit that in another more shaded area, another N. ‘Beetroot’ also of flowering size, has a much richer purple and is less blue blush in the centre but has less pronounced banding. This second plant is growing in about 70% of full sun light for the same period.
Unfortunately, living in the tropics the sun is not always to the north or directly overhead as it is when I am writing this. I have shade to the south so soon some of these plants will go into shade. They have had intense light since last summer and are hardened. This upcoming period of softening in full shade will be followed by sudden summer sun exposure as the sun moves back north. This will be the testing time for these plants. Only a wet season might protect them. It will confound my southern visitors once more. This garden is very wide, probably 4 metres. They will ask, “Why are the plants on the northern side and in full sun doing so well while the ones to the back are showing signs of stress? Next year there should be no problem as I will have two tree ferns in for horizontal shade. Paw paws make good temporary shade. The current vertical shade to the south will no longer be a problem.
As a final note, I would like to mention that the presence of carotenoid pigmentation, mentioned above, does detract from the sharpness of colours but it is not harming the plant. Beta-carotene in fact does just the opposite. It absorbs very much the same radiation frequencies that the blue part (high energy part) of the light spectrum that alpha-chlorophyll absorbs. You can consider it as a plants sunscreen protecting chlorophyll from over exposure. This is why green plants conditioned to high light survive while soft ones burn and succumb. As long as chlorophyll survives the yellow carotene pigment can take over some of the chlorophyll’s role of trapping light energy to allow photosynthesis. It can’t do it on its own. Any school botany book will satisfy your search for further knowledge in this area as it is the same for most other plants.
References
1 Benzing,D. H And Renfrow. Bot. Gaz 132(1) 19-30 1971
2 Neales. T. F. Australian Journal of Biological Sciences 26:705-714,1973)
3 Hartsock,T.L. and Nobel, P. S. Nature 262:574-576.
· Note: Units of light intensity (flux)— two different sets of units are popular. For conversion —1 Foot Candle = 10.76 Lux