When writing a book, his object was to obtain the best possible information on the subject he had in hand; he was never deterred from seeking this by any considerations of trouble or expense.

That these feelings influenced him from the time of his first arrival in England, may be traced in every page of his Journal; that they continued to influence him to the last day of his life, was only too evident to every one around him at that mournful period.

When Mr. Loudon first arrived in London, he was very much struck with the gloomy appearance of the gardens in the centre of the public squares, which were then planted almost entirely with evergreens, particularly with Scotch pines, yews, and spruce firs; and, before the close of the year 1803, he published an article in a work called The Literary Journal, which he entitled "Observations on Laying out the Public Squares of London." In this article, he blamed freely the taste which then prevailed, and suggested the great improvement that would result from banishing the yews and firs (which always looked gloomy from the effect of the smoke on their leaves), and mingling deciduous trees with the other evergreens. He particularly named the Oriental and Occidental plane trees, the sycamore, and the almond, as ornamental trees that would bear the smoke of the city; it is curious to observe how exactly his suggestions have been adopted, as these trees are now to be found in almost every square in London.

About this time, he appears to have become a member of the Linnaean Society, probably through the interest of Sir Joseph Banks, to whom he had brought a letter of introduction, and who, till his death in 1820. continued his warm friend.

At the house of Sir Joseph, Mr. London met most of the eminent scientific men of that day, and the effect produced by their conversation on his active mind, may be traced in his Journal. Among many other interesting memoranda of new ideas that struck him about this period, is one as to the expediency of trying the effects of charcoal on vegetation, from having observed the beautiful verdure of the grass on a spot where charcoal had been burnt.

In 1804, having been employed by the Earl of Mansfield to make some plans for altering the Palace Gardens at Scone, in Perthshire, he returned to Scotland, and remained there several months, laying out grounds for many noblemen and gentlemen. While thus engaged, and while giving directions for planting and managing woods, and on the best mode of draining and otherwise improving estates, several ideas struck him, which he afterwards embodied in a book published in Edinburgh and in London. This, then, was the first work of Mr. Loudon's presented to the public through the Messrs. Longman, with whom he continued to transact business of the same nature for nearly forty years. The book alluded to was entitled "Observations on the Formation and Management of Useful and Ornamental Plantations; on the Theory and Practice of Landscape-Gardening, and on Gaining and Embanking Land from Rivers or the Sea." This was his first separate work, and shows how strongly his mind was, even in his youth, imbued with the subject of his profession, though he was then apparently disposed to treat it in a different style from what he did in after years.

The work is divided into sections, in one of which, in particular, on the principal distinctions of trees and shrubs, are some very interesting observations, which show how well their author was acquainted with the characteristics of trees and shrubs even at that early period of his life. Before Mr. Loudon left Edinburgh, he published another work, entitled "A Short Treatise on some Improvements lately made in Hothouses." This was in 1805; and the same year he returned to England. On this second voyage to London, he was compelled, by stress of weather, to land at Lowestoffe; and he took such a disgust at the sea, that he never afterwards travelled by it, if it was possible to go by land. He now resumed his labors as a landscape-gardener; and his Journal is filled with the Observations he made, and the ideas that suggested themselves of improvements, on all he saw. Among other things, he made some remarks on the best mode of harmonizing colors in flower gardens, which accord, in a very striking manner, with the principles afterwards laid down by M. Chevreul in his celebrated work, entitled De la Lai du Contrasts simultane des Couleurs, published in Paris, in 1839. Mr. Loudon states that he had observed that flower gardens looked best when the flowers were so arranged as to have a compound color next the simple one which was contained in it Thus, as there are only three simple colors - blue, red, and yellow - he advises that purple flowers, which are composed of blue and red, should have yellow next them; that orange flowers, which are composed of red and yellow, should be contrasted with blue; and that green flowers, which are composed of blue and yellow, should be relieved by red.

He accounts for this on the principle that three parts are required to make a perfect whole; and he compares, the union of the three primitive colors formed in this manner with the common chord in music; an idea which has since been worked out by several able writers. He had also formed the plan of a Pictorial Dictionary, which was to embrace every kind of subject, and to be illustrated by finished wood-cuts printed with the type.

In 1806, Mr. Loudon published his "Treatise on Forming, Improving, and Managing Country Residences, and on the Choice of Situations appropriate to every Class of Purchasers. With an Appendix containing an Inquiry into the Utility and Merits of Mr. Repton's Mode of showing Effects by Slides and Sketches, and Strictures on his Opinions and Practice in Landscape-Gardening. Illustrated by Descriptions of Scenery and Buildings, by References to Country-Seats and Passages of Country in most Parts of Great Britain, and by thirty-two Engravings".