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San Diego Home/Garden Lifestyles"Much more than just another tree encyclopedia."
—San Diego Home/Garden Lifestyles, June 2005
—San Diego Home/Garden Lifestyles, June 2005
The most important thing to bear in mind is the situation for which the trees are intended to grow. Tho often people make a hurried choice based purely on the appearance of a plant itself with no regard to its attributes, requirements, or the function it needs to fulfill in the garden. Just like the beguiling puppy in the pet store window, purchases made in haste can have long-lasting consequences, especially when that small sapling shoots to over 80 ft. (25m) and outgrows the garden. It is much better to take time to decide what the tree is wanted for so that a more objective decision can be made to match the garden's requirements to the attributes of a particular plant. The final choice may still have an element of emotional impulse, but it should at least be based on a shortlist of suitable candidates.
The first questions to ask are to do with what the tree is needed for and whether there are any particular attributes required. Once this has been determined a list of plants that meet those criteria can be drawn up. Finally, the list can be refined to include only those trees that will tolerate the particular climatic, soil, and other conditions encountered in the intended location. Factfinders and selectors are particularly useful in this process.
Although books and catalogs can provide ideas, facts, and figures, there is nothing like seeing the plant "in the flesh," and visits to botanical gardens and arboreta are invaluable for checking out selected trees before purchase.
Trees play a very important role in the landscape of almost all gardens. They provide the structural framework below which the smaller and more ephemeral plants can be arranged. When designing from scratch they are often the first things to be sited, but for most of us inheriting an established garden, we must, to some extent, choose trees to complement the existing features.
Whether it's an individual plant that is required to meet a particular need, or a number to provide an overall structure, it is important to consider how they will contribute to the variety of colors, textures, and shapes to create a varied and interesting atmosphere. If a garden is to be more than just a jumble of plants it is also important that they are arranged so as to complement each other and show off their attributes to the full. Seasonal plants should be chosen and positioned carefully to provide a sequence of interest and brighten up even the dullest times of the year. Whether it be flowers, stem color or fall fruit, by careful combination it is possible to ensure that as one plant's display fades, another is about to step into the limelight. For some, like Amelanchier, the floral display is short-lived, and for the rest of the year it relies on the modest beauty of its foliage for appeal. Others, such as Betula pendula 'Youngii,' have all-year-round impact and should be given pride of place in a prominent position. In very small gardens with only enough room for a few plants, trees like this are particularly valuable. A few species are of such unusual or dramatic appearance as to make a particularly bold landscape statement. Purple-foliaged plants such as Cotinus coggygria 'Royal Purple,' and those with highly unusual leaves, like Trachycarpus fortunei, fail into this category. It is always tempting to fill the garden with plants like this in an effort to ensure constant spectacle. As in most things, you can have too much of a good thing and such a policy inevitably results in a devaluing of even the most spectacular plant. They should be used very sparingly, and most small gardens arc unlikely to have room for more than one or two.
Some kinds of plants play a more supportive role in the garden. Evergreens, such as yews and hollies, may not be the most dramatic of plants in themselves, but for much of the year are the great unsung heroes of garden design, providing backgrounds to enhance more ornamental species and screens to control views. They may have an even more down-to-earth function as hedges to define boundaries, or shelter planting against cold winds.
Of all the characteristics to be decided on when choosing trees, size must be the most important. As well as the obvious need to avoid species that will outgrow the garden, it is important that they are in scale with the space available and complement the other plants. Even trees that reach the modest height of about 33 ft. (l0m) are too large for some spaces, while smaller individuals of 6 to 9 ft. (2 to 3m) would look out of proportion if grown as specimens in larger settings.
As well as height, spread is an important consideration. Parrotia persica, with a typical height of about 26 ft. (8m), may seem a better choice for a confined space than Juniperus scopulorum that may reach 40 ft. (12m). However, the latter's narrow form allows it to be accommodated in gardens where the spreading habit of Parrotia would soon become a problem.
One difficulty with predicting plants' eventual size is their great variability. Even individuals of the same species show marked differences depending on climate, soil conditions, and origin. Magnolia grandiflora frequently reaches a height of over 80 ft. (25m) in the southern states of the USA but is restricted to much more modest proportions when planted further north. It may be possible, particularly with more common trees, to look around in neighboring gardens to see how large a particular species is likely to get. However, if in doubt it is best to err on the side of caution and choose something smaller. Fortunately, many cultivars have been selected that provide scaled-down versions of species. Magnolia grandifiora is a good example, with 'Little Gem' among its many smaller cultivars.
Having determined the physical and aesthetic attributes needed to complement the garden landscape, it is important to ensure that the trees chosen will succeed in the climatic zone for which they are intended. Even the most beautiful species grown in unsuitable conditions can turn out to be an ugly and disappointing specimen.
Hardiness is one of the most important considerations and may impose severe limitations on the list of possible species. It is useful to refer to climate zone maps such as those at the back of this book. These provide guidance based on the minimum winter temperatures likely to be encountered in each zone, and can be used in conjunction with the figures given for each plant in the selector. However useful these are, they cannot hope to encompass the more subtle variations in conditions that occur locally and should not he used religiously. Some plants, like Azara microphylla, may be grown in zones below their quoted figure with the shelter of a wall, while others that should in theory be hardy in a particular area may prove not to he due to spring frost patterns. An additional factor to hear in mind is that plants adapted to cool conditions often do less well in warmer climates. This should be considered carefully where plants are intended for zones well above their minimum.
Watering the garden is a chore that most people would prefer not to have to do. Occasional supplementary watering during dry spells is one thing, but it is best to avoid planting trees that have requirements well in excess of the natural rainfall for an average year. In dry climates it is best therefore to select trees like Robinia pseudoacacia 'Frisia' that have the ability to survive periods of drought. Again, selectors are useful for helping to determine which plants are suitable.
Trees have evolved in a variety of different wild environments. Acer palmatum, for example, grows naturally in the forests of central China, Korea, and Japan, commonly in the partial shade of larger trees. This characteristic can be used to advantage in the garden where it can occupy shady situations or form an intermediate layer between larger trees and herbaceous plants. Some trees, on the other hand, require full sunlight if they are to thrive or flower well. Most willows are known for their dislike of shade, and Koelreuteria paniculata rarely flowers unless given full sunlight.
Shelter is also an important factor determining the success of some species, particularly in their young stages. Tender trees like Embothrium and Crinodendron may be grown successfully in areas outside their normal climatic limit with the shelter of a protective wall or adjacent plants. Others such as Stewartia and some species of Nyssa benefit from shelter even within their climatic zones. Some of the most useful species are those that thrive even in quite severe exposure and can he used as windbreaks for humans and other plants alike. All species of Crataegus come into this category, as do most birches and pines.
Like all plants, trees vary greatly in their soil requirements, and it is usually their tolerance to various adverse conditions rather than strict requirements that limits their success. When we say, for example, that a particular plant requires an acid soil, it is more accurate to say that it cannot tolerate alkaline conditions.
Soil type is a fairly loosely used term that refers to the proportions of sand and clay in the soil. This has a strong influence on its ability to retain water, drainage, and fertility. Some trees, such as pines and birches, thrive in the relatively poor, dry conditions encountered on freely drained sandy soils. Others such as Stewartia have more demanding tastes, requiring a moist but well-drained loamy soil. Soils that are constantly wet or liable to periodic waterlogging, particularly during the growing season, require specialist species. Willows and alders are the most obvious candidates for these conditions, but less well-known ones include Betula pendula and Sorbus aucuparia.
The pH of a soil is a measure of its acidity or alkalinity. The scale runs from 1 to 14 with 7 being neutral, and acid and alkaline being lower and higher respectively. In general, alkaline soils tend to be more fertile, but at high levels (above about 7.5) nutrients become unobtainable by many plants. This is why tolerance of high pH is such an important factor in determining the suitability of plants for lime soils. Acid soils tend to be poorer as nutrient elements are freely dissolved and leached out. In practice, alkaline soils impose a greater constraint on species choice than acid ones. However, there are plenty of species that will thrive even in quite severe alkaline conditions, and trees are often more tolerant than they are given credit for. The ultimate solution for accommodating a "must-have" tree — despite unsuitable soil conditions — is to grow it in a container with its own particular taste in compost.
Some trees are able to thrive on almost any kind of soil and provide an obvious choice where conditions are less than ideal. Rhus typhina is one such plant, a fact that explains its great popularity and success.
Trees suffer from a multitude of pests and diseases, requiring a specialist book to do the subject credit. Most are sporadic in distribution and occurrence and need not be considered when choosing trees for a garden. Others, such as honey fungus, are common and serious and, where established in a garden, may influence choice in favor of more resistant species.
A few diseases are sufficiently severe and prevalent to render certain trees completely unsuitable for some areas. An obvious example is fireblight, a bacterial disease that affects those genera of the rose family that possess apple-like fruits, including Pyrus, Sorbus, Crataegus, and Malus. As well as being very damaging to ornamental trees and shrubs, it is a commercially important disease in fruit-growing areas. Where plants susceptible to diseases such as fireblight are chosen for planting, advice on their suitability for use in a particular area should be sought from local arboricnltural or agricultural advisors. In addition, disease-resistant varieties should be used where possible.
Of the many factors to be considered when choosing trees, the danger of them spreading from the garden by seed and sucker to become invasive weeds is one of the most important. There are many well-documented cases of plants and animals becoming major pests in some areas. The European holly, Ilex aquifolium, thrives in the cool, moist conditions encountered in the northwestern states of the USA, and has spread into forests at the expense of the native vegetation. Government agricultural and environmental departments produce lists of potentially invasive species and these should be referred to before planting species that have the ability to spread.
Some trees have toxic leaves, fruit, or seeds, while others are armed with thorns. Where trees are being chosen for planting close to areas used by the public, and especially children, these factors should be borne in mind and will make certain species unsuitable.