The wall environment is quite harsh. There are a few bonuses but mostly the species living there will have to have special adaptations to cope with difficult conditions.
Depending on the construction of the wall the availability of water will be a bigger or smaller problem but it will always be a problem. Dry stone walls will potentially be able to take in more water than a solid construction especially if the central region has been filled with soil as well as stones. The older the wall is the greater the amount of organic matter inside and this will absorb and retain water quite well, but always there will be a propensity to dry out especially during a hot dry summer. A south facing surface will lose the most water but quite quickly an entire wall can dry out. There will be a certain amount of absorption from the soil below even without any rainfall but this will not amount to much in midsummer.
Consequently plants trying to colonise walls will have to be xerophytes. Xero translates as dry and phyte as plant so xerophytes are plants such as cacti and succulents which are adapted to dry conditions.
Succulent leaves of Stonecrop.
Reduced leaf surface area such as is found on Stonecrops reduces water loss. Hairy or shiny leaves also have this effect. The shininess is caused by a thick outer cuticle which prevents water loss. Less obviously but equally important is a large root system that can penetrate and get down through the wall to make use of any damp patches. A root that can store water might also help, so a large tap root might see plants through a bad patch.
A large penetrating root system will have other benefits such as helping to improve the plants uptake of fertiliser and minerals, which might be in short supply. It will also serve to anchor the plant firmly into the wall so that it will not be blown out or washed out during inclement weather.
Nutrients, especially nitrates and phosphates which normally come from the breakdown of organic matter may be in short supply. Any insect or spider entering the wall and then dying there will become a source of nutrients as will dead plant material such as dead roots from previous generations of plants and dead mosses and lichens which might have colonised the surface and crevices and cracks. However this is going to provide a relatively small input compared with what goes into soil. Some plants can fix nitrogen in their roots. They have little swellings called root nodules and these contain bacteria which convert atmospheric nitrogen into nitrates which is all very useful to the plant but this feature is restricted to plants in the the pea family so for most plants the option is not available. It is perhaps surprising that few pea species grow on walls. Kidney Vetch is an exception.
Kidney Vetch growing from a harbour wall.
Another factor to consider is whether the wall is acidic or alkaline. This will largely be determined by the material used in the construction of the wall. Limestone walls will be alkaline. Brick walls will also be alkaline partly from the bricks themselves but particularly from the mortar used. Efflorescence sometimes occurs on new walls. This is caused by salts which are alkaline sulphates such as sodium and potassium. Sources of these salts include Portland cement, lime, sand ,which may have been sourced from the sea shore, and clay used in the brick which may have come from saline earth. On the other hand walls made from slate, sandstone and granite will be acidic. Rainwater is also slightly acidic so the top of a wall and the side exposed to the rain will always be a slightly lower pH than the rest of the wall.
Plants may have different preferences for pH. Some favour alkaline and are called calcicoles, others like acidic and are called calcifuges but most are not too fussy as long as it is somewhere close to neutral so the type of plants growing on walls will be somewhat influenced by the material used in the wall’s construction.
Then there is the position, whether the wall is facing north or south will have an impact, more light on the south side but dryer. Walls facing south west in Wales and Devon and Cornwall will catch more rain, in East Anglia they would be better off facing north east. Growing lower down the wall will mean more water available but might mean more competition from plants growing next to the wall especially if it is a tall crop like Maize or Sunflowers. Growing higher up or on top means more light but more exposure to the wind and probably less water.
Many wall plants are annuals or even ephemerals. Annuals only live for a year, ephemerals are short lived plants. Many species found on walls germinate in early Spring, grow and flower by May and then produce seed and are dead by mid June, thereby timing their life cycle so that they can exploit the wet and reasonably warm months and avoid the hot dry summer period.
So life is going to be hard for the wall plant which no doubt explains why my list of wall species is relatively short.
Walls and Flowers 