Photons are the particles of energy coming from the sun that plants use to photosynthesize sugar. It is this energy source that runs the plants entire metabolism. Plants trap them in chlorophyll (the green pigment) and pass the energy along to fuel their various processes.
When sunlight directly strikes a leaf, 93% of the photons pass right through it. The leaf only captures and utilizes about 7% of the photons striking it. The rest, however are still available to strike a leaf below the first one. That is why some plants are able to have many layers of leaves – there is nothing wrong with leftovers!
The VERY FEW layers of a Taxus hedge.
SHADE vs SUN
Eventually, not enough photons reach the lowest leaves of a forest (or the inner leaves in a hedge) to make a difference. In general, the maximum number of layers of leaves in a plant is about 6 or 7 – but this very species dependent. When not enough photons reach that far down (or in) it is no longer viable, and the leaf is dropped.
This fact helps explain what we call Full Sun, Partial Sun and Shade plants. The tolerance of the plant to how many photons it receives determines the category we put it into. Those that tolerate shade can manufacture enough sugar to sustain their metabolic processes from the limited quantity of photons they are receiving – think forest floor.
Shade plants at the forest floor.
Some plants are intolerant to low photons when placed in a shady area. They must receive a high number of photons to produce sugar. We call those Full Sun plants. Pine trees are a good example, boxwoods too !
All our Design Team members are well versed in which plants do best in Sun or Shade !
Hedges are groups of plants in which their outer layers are in Full Sun and their inner layers are in shade. The higher the amount of photons available the thicker the hedge can get. Likewise, plants in low light form very few layers – think Hosta.
Our Install team is well aware of these issues and keeps low light plants out of the sun during an installation.
See these Hosta loving the shade!
Some broad leaf plants that are tolerant to lower light will adapt their new leaves to compensate for the reduced number of photons available. Their new leaves become thinner and wide, allowing them to capture more photons. One consequences of thinning the leaves is the leaf strength itself. Eventually the leaf can’t get any thinner, nor sustain itself upright – it simply flops down.
The same Beech tree with leaves progressing in size from the top (full sun) on the left to the base (shade) on the right.
Another big problem of broad leaf plants that adapt to low light is they no longer produce enough wax on their surface. This is a because they no longer needs that protection from the sun – the main function of leaf wax. However, it is also due to the lower sugar production – as sugar is what the plant uses to make wax.
These thinner leaves tend to be very susceptible to leaf fungus (mildew). A thinner wax layer enables fungi to penetrate them and infect them. It is a very common problem of lilac when placed in the shade.
Mildew is such a fungus that targets Lilac plants in low light.
Some plants have leaves that simply wont modify in lower light. All needled evergreens (pines, junipers, cedars) fall into this category. They did not evolve the ability to modify the leaf structure to become thinner and wider. They simply perish once all the stored energy is consumed. Pines for example evolved in full sun where it had windy, hot, and dry conditions – hence their long tubular leaves covered in wax !
This blue spruce prefers full sun to get its blue color!
If you want to have plants in a shady garden – Call Plant Specialists TODAY !
We have a Design Team that knows which plants to use and can install them for you ! !
Don’t delay – the sooner the better !
GREENING NEW YORK FOR OVER 51 YEARS !
Article written by our Staff Horticulturist, Peter B Morris, BSc, MSc, MBA
All photographs used with permission @SHUTTERSTOCK