Monday, 1 December 2014

Plants, light intensity and the camera

 Gardening and photography have rather a lot in common, in that they are both very dependent the availability of good light.

Plants grow best when they have plenty of light and cameras produce their best photographs under the same sort of conditions. If there is not enough light, plants become yellow and elongated, and cameras produce photos that are fuzzy or too dark.

In gardening there is always a lot of discussion of the business of "enough light" but rarely any clear definition of what constitutes "enough". There are phrases like "full sun" and "partial shade" but these refer to the number of hours in the day during which a given area of the garden will be bathed in the maximum available sunshine. This takes no account of the strength of sunshine available, which may vary significantly depending on the time of year and the location of the garden.

So here's a question: Can we measure the intensity of sunlight using a digital camera? I don't mean with a level of accuracy that runs to the nearest photon. What's needed here is just accuracy sufficient to support meaningful discussion of gardening conditions between people who may live in very different parts of the world. For example, could a gardener in Egypt describe his gardening conditions to a Canadian gardener by using his digital camera to take measurements of light intensity?

I've been experimenting with this a bit and here's one of my first attempts.

This photo was taken at the end of spring (29th March) with the following parameters:

1/80th second exposure

The photograph below was taken with the same camera, lens, and location, but on the first day of December, of the same year. 

1.3 second exposure

So can we calculate the difference in light intensity based on these parameters? To make it easier I have taken the second photograph using the manual setting on the camera, and I have set the ISO and aperture to the same levels as in the first photograph. 

I had to use a tripod to take the photograph, as the only paramater I wanted to vary was the exposure length. I ended up needing quite a long exposure, which would have introduced a lot of camera shake without a tripod. 


The answer is - Yes! It's actually pretty easy to make a direct comparison of light levels using a camera. However, it really helps if the same model of camera and lens is used to measure both samples, and it really very much helps if the camera has a manual mode to enable some parameters to be kept constant between both tests. 

The bottom line though, is that if you want to talk gardening with your distant friends, even friends who may currently be in a different season (perhaps Australia versus UK) or in a different latitude (e.g. Northern Scotland versus Botswana) then it's pretty easy to do it. 

I don't think we need to worry about expressing light intensity in Lux, or photons or anything tricky like that. As gardeners we could probably stick to expressing it in seconds of camera exposure time, on the understanding that all other parameters are kept the same between our location the location for comparison. 

The next question is to ask how this works with different models of camera, but that is a whole other thing. A question for another day I think. 

An add-on - 

MaryWilliams @PlantTeaching on Twitter has suggested that this work might also be done using a smart phone app. I've had a quick look online and it seems that there are already a number of light meter apps available. If you try this, please do write and let me know how it goes. I will compile the results from different locations. 

Good luck!

Wednesday, 17 September 2014

Bird Cherry Ermine

The Bird Cherry Ermine (Yponomeuta evonymella) do an amazing thing in the park near us every year. They colonise a whole row of Bird Cherry (Prunus padus) trees, eat all the leaves, and drape the trees all over in white webs, so that the trees resemble snowy Christmas trees. Below are some photos of the most recent colonisation.

Initially the trees look just like any other tree in summer - covered in leaves. Then the eggs are laid on the trees, and hatch into caterpillars. The caterpillars are there in their thousands and eat all the leaves completely away, so that not a single fragment of leaf is left.

In the distance the trees just look white. 

In close-up we can see the thick white webs on the twigs of the tree, and the caterpillars swarming all over them.

At the base of the tree, the webs fan out and the caterpillars can be seen marching away towards, what they hope is the next tree. They usually have to dodge round a bunch of curious children and lots of adults with cameras.

In the photo below you can see a grey area on the base of the trunk. This is a big pile of caterpillars. There are thousands of them on each tree.

The caterpillars do this for maybe a month or two each summer, and then they all vanish away to get on with their lives, and all of the trees' leaves grow back. A few weeks later, there is no sign at all that the caterpillars were ever there.

Tuesday, 25 March 2014

Growing Fern Gametophytes

This is a gametophyte of the fern Dicksonia antarctica. It is only about 1cm across and is composed of a heart shaped sheet of cells that is only one cell thick. 

A gametophyte of Dicksonia antarctica - copyright Jennifer Deegan.

Fern gametophytes are easy to grow. You need a clean pot filled with potting compost. First scald the compost by pouring boiling water through it. This kills off any fungi that could harm the gametophytes. 

Scalding soil before sowing fern spores.

Next allow the soil to drain and cool completely to room temperature. Once the soil is cool, sprinkle the fern spores on the surface of the soil. Then enclose the whole pot in a large freezer bag. Inflate the bag by blowing into it, and knot it at the top. The picture below shows a blue bag, but the bag must be transparent, and should ideally be colourless.

Enclosing the pot in a bag to give a humid atmosphere and to avoid needing to water the pot.

Put the whole assembly somewhere at room temperature and check occasionally for growth. The pot will not need watered, and the bag will not need to be removed until the sporophyte plant is well developed. The gametophyte will be visible within a couple of months.