The challenges of coastal gardening
We have seen why people like to live near to the sea, but enjoying this way of life does not come without its problems. Gardeners, particularly, must account for the vagaries of the climate, and it is not unheard of for people to move to the coast, find it is not to their liking, then move back inland. This chapter is designed to alert you to the problems before you make your decision to move seawards.
The aspect of the weather that tends to affect the lifestyle of coastal dwellers the most is the wind. In meteorological terms, there are two basic types of wind you will find at the coast:
• A ‘pressure wind’ – created by air movements from high and low pressures. There are four basic pressure types: anticyclone, ridge of high pressure, depression and trough of low pressure.
• A sea (or ‘onshore’) breeze – this blows from the cool sea surface on to land, where the air is warming and rising during the day. By mid-afternoon, a sea breeze can spread as much as
15–30 miles (24–48km) inland. At night sometimes the reverse can happen; the land cools and the sea can be warmer, so air travels from the land and towards the sea (known as an ‘offshore’ breeze).
Coastal wind speed depends on the sea breeze and the pressure wind. Wind near the coast is stronger than elsewhere if the two components act together in the same direction, but is less if the two components oppose each other.
A sea breeze, bringing cold air inland, may confront an area of warmer air over the land which is moving in the opposite direction. A roll of shallow cloud often forms at the junction, which is known as a ‘sea breeze front’.
Shelter is essentially provided by vertical structures, and these fall into three different categories – walls, fences, and natural screens and hedges – with a number of subcategories.
There are two types of stone wall. A ‘dry-stone wall’ is made without mortar and is usually well suited to rural and coastal situations. The other type is the mortared stone wall, which is stronger and able usually to retain soil, at least for part of the way up. These mortared walls do not look as rustic, but they can be made to look more like a dry-stone wall by ‘raking’ out the joints.
Brick and block walls are more commonly used for buildings than garden features or boundaries, but they can be very effective in a decorative sense. Block walls, which are much cheaper to build than brick walls, are usually rendered to produce a surface that is smooth and attractive, which can then be painted.
The only problem with having a rendered wall close to the sea is that the constant buffeting by wind – and the inevitable salty corrosion – means that the rendered surface can crack and peel. Scraping, filling, smoothing and painting may be necessary every one or two years, depending on the age of the wall and the prevailing conditions.
Screen-block walls became very popular in the 1970s, and they have never really gone away since. These ‘see-through’ patterned blocks are attractive and useful, but they produce a weak wall in comparison to solid brick, block or stone, and for this reason a screen-block wall should be strengthened with several supporting piers.
A fence is not going to be as durable as a properly built wall, but you may decide to choose a fence for financial reasons or because, aesthetically, a fence is more appropriate to the garden.
Timber panel fencing is the most common type. This could be overlapping horizontal larch strips, interwoven strips, or close-boarded fencing using vertical feather-edged boards. These panels are joined together via stout wooden or concrete posts; the former may last just a few years in a very exposed coastal situation, whereas the latter should last for many years.
Metal fencing is commonplace too. Wire fences include chain link, chicken wire and ‘livestock’ fencing. There are also steel fences in the form of decorative wrought iron and vertical railings. These last two are expensive in comparison, but can look magnificent in the right setting. Be aware, however, that metal corrodes and rusts much more quickly where there is salt in the air, so be prepared for high annual maintenance.
These are panels made from woven willow and hazel (known as ‘hurdles’), or bamboo, reed and sometimes even heather. Split bamboo screens also come in rolls. In all cases the panels are wired to rustic posts, and they make quick, easy and relatively cheap screens. However, they are nowhere near as permanent as the other types of fences mentioned, nor as durable.
Windbreaks and shelter belts
By far the best way of reducing the impact of the wind, if there is space, is to put up a windbreak to filter the wind.
An effective windbreak should be high; it is estimated that a windbreak 20ft (6m) high offers wind protection to a man standing 50ft (15m) away. Also, the denser the planting, the better the protection. Depending on the situation, even a low shrub border just 10ft (3m) high and comprising a variety of bushy plants can be effective. A windbreak should preferably comprise evergreen subjects so that there is year-round leaf cover, but deciduous trees and shrubs in their dormant season can still work very well at reducing wind speed.
Windbreaks are certainly better at reducing wind severity than solid walls and fences, as these can create turbulence and ‘eddies’ by forcing the wind up and over them, to descend again on the leeward side. Such turbulence can damage any plants that are within the vicinity.
If you put up a wide band of trees and shrubs, two or more deep, then this is more often referred to as a ‘shelter belt’. If substantial trees are your choice for a shelter belt, you should be prepared for a wait. In the mid-1800s Osgood Mackenzie, owner and founder of Inverewe in the north of Scotland set out a belt of the Monterey pine (Pinus radiata) – then left them for 25 years before planting his garden! However, it might take just a decade or so for a belt of year-old trees or saplings to develop into a useful shelter belt.
The following conifers, in addition to the Monterey pine, make fine subjects for shelter belts: the beach pine (P. contorta), the lodgepole pine (P. contorta latifolia) and the Monterey cypress (Cupressus macrocarpa) – but the latter should be left unclipped, as it does not make a good trimmed hedge. The two best deciduous trees are ash (Fraxinus excelsior) and sycamore (Acer pseudoplatanus), although in the early years both may be slightly shaped by the wind.
A gardener should always have at least one hedge to look after. Planting a hedge and maintaining it are both ‘real’ gardening jobs, requiring skill and insight. Painting fences with preservative, or wrought iron with rust preventative, is a do-it-yourself property maintenance job! Additionally, a mature hedge can look wonderful, filter the wind magnificently and be a haven for wildlife.
In the case of a coastal garden you will need to make sure that the plants chosen for the hedge are suitable for battering from wind and salt; if there is space, you may like to consider planting a shelter belt on the windward side of the hedge – this really will give you the best outlook horticulturally.
As for suitable plants, amongst the best are Griselinia littoralis, holly (Ilex aquifolium), evergreen oleaster (Elaeagnus x ebbingei, E. pungens and its variegated forms), Escallonia, Olearia, Euonymus japonicus, Pittosporum and Brachyglottis.
In exposed gardens light items of furniture can be blown over – or across distances. It makes sense to anchor these down somehow, or to stack them and put them in storage if bad weather is forecast. Of course, the heavier the furniture is, the less movement there will be.
Many suites of garden furniture these days come with pre-made covers for use during off-season periods. If these covers are used, and they encompass, say, a table and its chairs, and the cover is firmly fixed in place, there is less likelihood of wind movement and the whole suite should be secure. However, it is always better to put furniture into winter storage, if this is available.
The problems of salt
Sea water contains approximately 3 per cent salt as well as other minerals. However, the proportion varies considerably, according to the ground over or through which the water has been fed.
The sea with the highest salt content, and therefore the highest density, is the Dead Sea between Israel, the West Bank and Jordan. It is so named because nothing will live in it; normal saltwater fish species die as soon as they are put into it. It is actually a large inland lake, some 1,300ft (394m) below the level of the nearest open sea, the Mediterranean. It is the lowest stretch of water on the planet and contains about 25 per cent salts, of which 7 per cent is common salt. Understandably, no vegetation of garden merit grows along the banks of the Dead Sea; you would be hard-pressed even to find any highly salt-tolerant reeds and grasses.
Generally, sea water freezes at a lower temperature than fresh water, at between –1.5° and –2°C (29° and 28°F), depending on the degree of salinity. As it freezes it separates and becomes a mixture of freshwater ice, brine and air. Water just below sea ice may be more salty than the rest. Estuary water is a mixture of sea water and fresh river water and is therefore considerably less saline than open sea. Consequently, estuaries freeze more readily than the sea.
Salt-laden sea water causes problems for plants in two ways: salty air and seawater flooding. Let’s take the most common of the problems first.
How does the air become salty? Out at sea, the wind will whip up droplets of water, and the smallest, finest ones are carried inland on air currents. As they travel inland, some of the water evaporates and this makes the remaining solution more salt-concentrated. These droplets can be held aloft for several miles before settling, and of course this may be on any surface, from soil and plants, to cars, buildings, animals … anything.
Unless washed away by rain, or water from a sprinkler or hose, the salt level will build up and this accumulation of deposit is, in the main, what causes damage to plants. The degree of salinity in the air varies hugely depending on four variables:
• The salinity level of the sea
• The wind speed and direction
• The mean air temperature
• The distance travelled.
As for this last variable, it has been recognized that salt spray is most damaging to plants growing within 1,000ft (305m) of the shoreline.
Coastal garden owners in some parts of the world, including the US Eastern Seaboard, are fortunate in that the worst of the storms come in late summer and autumn, when there is not so much in the way of new, emerging growth and buds to be damaged. But the news is not so good for gardeners in the south and west of England and Wales, and the west coasts of Scotland and Ireland, as the westerly winds regularly batter gardens that have been brought on earlier by the flow of warm waters via the Gulf Stream.
Salt can also be deposited by sea mist and fog (see page 37) which closes in around plants. When the air is still, droplets of salty water settle on leaves and stems. The water evaporates when the mist clears, leaving a dry salty covering that remains in place until it is washed away by rain, or the gardener when watering plants.
Salt air is not always a bad thing. It is well known that salt spray rids certain plants of insect infestations (roses and hollies reportedly have less aphid presence in coastal gardens), and it also helps to control fungal diseases (zinnias and lilacs do not so readily succumb to mildew, to which they are usually prone).
Sea water in the soil
The two ways in which sea water gets into the soil are fairly rare but, if environmentalists are to be believed, they will become more common over the next 100 years or so.
The first and more obvious way is through seawater flooding. Unless your garden leads down
to the tidal water’s edge, this is unlikely to be a serious threat. But there are freak occasions when tides can flood a beach and flow through gaps in the dunes, dikes or levees, and even pour right over gardens farther inland. The gardens most affected will be those in the lowest lying areas. It is claimed that the incidences of storms and floods (and in extreme cases, hurricanes and cyclones) will increase and become more severe as a result of climate change, and this should be borne in mind by anyone wanting to embark on creating a coastal garden.
The second way for sea water to get into the soil is less visible. During a drought the water table can drop so low that sea water gets into the ground water. When ground water becomes salty, plants draw up the salt via their roots, and too much can cause a withdrawal of moisture from the leaves, a process known as ‘exosmosis’. Plants become dehydrated and wilt (and eventually die).
Unfortunately the gardener usually thinks the problem is a lack of ground moisture, so applies more water to the soil. If this water is also salty, it just makes the problem worse.
Saline infiltration of ground water is becoming more problematic as the urbanization of seaside towns grows, which puts more pressure on finite water supplies.
The vast majority of plants recommended in this book will be tolerant of average salt-air levels. The only exceptions will be annuals, which last for just one or two growing seasons, and short-term vegetable crops, which do not stay in the garden long enough to suffer damage.
However, the more extreme the exposure to salt air, both in terms of the length of time of the exposure and the degree of salinity, the more damage there will be to the plants.
Typical damage will be a scorching of leaf margins, a powdery coating of salt on dry leaves, burnt tips to tender new growth in spring and, in severe and long-lasting cases, all of the above combined with a general wilting.
This damage is rarely to be found all over a plant, however. The side nearest the sea is likely to be in receipt of the most salt deposit, and growth on that side will slow down as the salt draws moisture from the plant’s tissues – leaves, buds and stems. The side of the plant facing away from the sea will have less salt deposited on it and will continue to grow more or less normally. Eventually the plant will become lopsided, appearing to lean away from the direction of the beach. This is the windswept look that is so typical of plants, trees especially, growing near to the sea.
Fortunately there are many plants that are highly or moderately tolerant of salt. Amongst the former are Agave, Hedera (ivy), Euonymus fortunei, some junipers, Liriope muscari (lilyturf), Magnolia grandiflora, Nerium oleander, Parthenocissus quinquefolia (Virginia creeper), tamarisks, thymes and Viburnum tinus (laurustinus).
Plants that are moderately tolerant of salt, but which are best when planted in sheltered gardens or behind windbreaks, include Agapanthus (African lily), Ageratum houstonianum (flossflower), Begonia semperflorens (bedding begonia), Calendula officianilis (pot marigold), Canna (Indian shot plant), Helianthus annuus (sunflower), Hemerocallis (daylily), Ligustrum (privet), Mahonia aquifolium (Oregon grape), Rosmarinus (rosemary) and Salvia officinalis (sage).
There are a few garden plants that are tolerant of seawater flooding, but they will usually only remain unscathed if flooding occurs during the dormant part of the year. These plants include Cytisus scoparius (Scotch broom), Elaeagnus, Salix (willow) and Sedum ‘Autumn Joy’.
Effects of salt on structures
In the garden there are several structures on to which it is not desirable to have a salt layer. Iron and steel, found in wrought-iron garden gates and patio furniture, are the most obvious examples. When in contact with water and oxygen, or other strong oxidants or acids, iron will rust; if salt is present, as in sea water, the metal rusts much more quickly.
Galvanization is an important approach to rust prevention and this typically consists of a layer of zinc applied to the surface of the object to be protected (by either hot-dip galvanizing or electroplating). Zinc is traditionally used because it is cheap and adheres well to the steel. In situations where salt water is present, however, cadmium is preferred. Galvanization often fails at joints, seams and holes, where the coating is pierced, so applying proprietary rust preventatives may be useful at these points.
Valuable metal objects such as cars and boats should not be left to the full force of the salty elements without being protected or covered. And steel fixtures such as metal roofs and gazebos should be regularly treated with rust preventatives.
In the case of wooden furniture, regular exposure to salt water, or salty winds, will cause the wood to dry out more rapidly than it would do otherwise. Without treatment it will crack and split, which will allow access to burrowing insects and harmful microorganisms. Oak (Quercus spp) and teak (Tectona spp) are dense, close-grained hardwoods high in natural oils. These characteristics give them strength and durability, but they still benefit from protective oils, applied with oily rags at the end of autumn and the beginning of spring.
Sea mist and fog
Most of us have seen rolling swirls of mist or fog coming off the sea, heading inland. Technically, this is known as an ‘advection’ fog, and it is formed when warm air cools over a cold sea. Because sea surface temperature does not alter much in the short term, the fog usually persists over the sea until there is a change in wind direction. It may be patchy as well.
Sea fog can exist in wind speeds of up to 15mph (24km/h) because turbulence merely brings more warm air into contact with the cooling surface. If the wind is any stronger than this, it will lift the fog off the surface to give very low cloud. Fortunately, any fog which drifts across warming land, particularly during summer, usually disperses a short distance inland.
Interestingly, in warmer places this sea fog can exist in even greater strengths. For example, on a sunny morning in Los Angeles, when the Mojave Desert further inland heats up and causes a sea breeze to blow, sea fog from above the cold Pacific to the west of California can be brought onshore like a tide. But it disperses relatively quickly, as it spreads over the warming landmass.
Coastal dwellers should be prepared for a greater-than-average presence of mist and fog than is experienced by those living further inland, and of course there will be a proportion of salt water – although not great – present within the sea fog. This is not usually of sufficient strength to cause any greater damage to plants than the normal salt content of coastal air.
This is a greater problem than you might imagine. For example, a catastrophic sandstorm occurred at Culbin in Scotland at the end of the seventeenth century. There had been a richly diverse estate and farmstead there for centuries, but by the 1690s the sandbanks and dunes along the shores of the Moray Firth had been exposed, as a result of local people stripping the grasses for use as roofing thatch. In the autumn of 1694, a deep depression caused a yellow blizzard of sand which lasted all night and drifted high against the houses. After a short lull, the storm renewed and the whole village was then submerged in sand, forming an arid desert covering an area some 8 square miles (21km2).
Occasional storms over the intervening years have reshaped the sand – sometimes exposing the tops of the still-buried houses. Since the 1920s the UK Forestry Commission has created plantations of grass, brushwood and young trees as a successful three-line defence against further wind and sandstorm damage.
In any coastal garden there will be a degree of blown sand. If the garden is particularly exposed or vulnerable to this, it would make sense to grow plants that are sturdy and able to withstand the onslaught. Many grasses are suitable, as the fine sand particles blow between the stems and blades, rather than against them.
Of course, even inland gardens can be covered in sand. In Europe sand falls from the sky occasionally, usually after being carried from the Sahara on high-altitude winds. But this is never in such a quantity as to damage plantings.
Access can be a problem for those owning coastal gardens. Historically, people who lived in small coastal villages and towns were poor and could not afford large houses, so dwellings were small and crammed together. Access for vehicles in some of England’s oldest coastal towns, for example, can be difficult.
Gardens right on the seashore may not have a service road at the back. Therefore in a new-build property full consideration should be given to access, both from the point of view of getting into the property and in terms of allowing space for small items of garden machinery, such as lawn mowers and wheelbarrows, to get from, say, the front garden to the back garden. See also Public or private beaches (page 19).
Heat and light
In coastal gardens the air is frequently clearer than inland, meaning greater light intensity and periods of intense heat in summer. High temperatures can induce several different problems with plants. One such is that of sun scald, which can arise when hot sun strikes the bark of thin-barked trees, such as beech (Fagus spp), cherries (Prunus spp), maples (Acer spp) and poplars (Populus spp). It can cause the death of the area of bark affected and, in severe cases, the whole tree.
The fruits of many plants grown for their produce may be similarly affected, such as tomatoes, apples, pears, gooseberries and grapes. If your garden is situated in an area known for its ‘good’ lighting in summer – such as the English town of St Ives (see page 15), which has been inhabited for centuries by artists who go there for its brilliant light and air quality – it may be safer to avoid using the plants mentioned above.