In 1836 he began The Suburban Gardener, which was also published, in monthly numbers, so that he had five monthly works going on at the same time He soon found, however, that three monthly works, besides the Arboretum, were as much as his health would permit him to undertake the management of, and he disposed of The Magazine of Natural history to Mr. Charlesworth. In 1838 he also gave up The Architectural Magazine, amid at midsummer in that year he finished the Arboretum Britannicum. He was now in circumstances that would have discouraged almost any person but himself. His health was very seriously injured, partly by what was supposed to be a liver complaint, and partly by an enormous swelling in his right knee, which some of the most eminent medical men in London supposed to be produced by a disease in the bone. In addition to the large sums in ready money he had paid to the artists and other persons employed during the progress of the Arboretum, he found at its conclusion that he owed ten thousand pounds to the printer, the stationer, and tile wood-engraver who had been employed on that work. His creditors, however, did not press him for their money, but gave him a chance of reaping the benefit of his labours at some future time, by consenting to wait till they were paid by the sale of the Arboretum and the Cottage Architecture, upon condition that he placed these works in the hands of Messrs. Longman, to hold for the creditors till the debt was paid.

Notwithstanding the state of his knee, which was now such that he was unable to walk without assistance, immediately on the completion of the Arboretum he arranged and published Ins Hortus Lignosus Londinensis; and in the last number of The Suburban Gardener, which was finished about this time, he informed the public that he intended to resume his profession of landscape gardener, and that he would not only go out, but give advice at home, on any plans that might be sent to him. To us, who saw the state of his health, this intimation gave the greatest pain, and we determined to do every thing in our power to prevent the necessity of his exerting himself. Two of his sisters learned wood engraving; and I, having acquired some knowledge of plants and gardens during the eight years I had acted as his amanuensis, began to write books on those subjects myself. In the mean time, he grew so much worse, that we had very little hope of his recovery, till he placed himself under the care of William Lawrence, Esq.; when that eminent surgeon took a different view of the case from what had been before entertained, and by his mode of treatment rapidly restored him to health.

In 1839 Mr. Loudon began to lay out the Arboretum so nobly presented by the late Joseph Strutt, Esq., to the town of Derby. In the same year he published his edition of Repton, and his Second Additional Supplement to the Hortus Britannicus. In 1840 he accepted the editorship of The Gardener's Gazette, which, however, he only retained about a year.

In 1840, Mr. Loudon, having a great desire to examine some of the trees in the Jardin des Plantes, in order to identify some of the species of Crataegus, went to Paris; and, as his health was beginning to decline, I went with him, taking with me our little daughter Agnes, who, from this time, was always the companion of our journeys. We went by way of Brighton, Dieppe, and Rouen, to Paris, ascending the Seine; and we remained in France about two months.

When Mr. Loudon left Scotland so abruptly in 1831, he promised his friends to return the following year, and, indeed, fully intended to do so; but various circumstances occurred to prevent him, and it was not till 1841 that he was able to fulfill his engagement. In the summer of that year, however, soon after the publication of the Supplement to the Encyclopedia of Plants, Mr. Loudon, Agnes, and myself, went from London to Derby, and, after spending a few days with our kind and excellent friend Mr. Strutt, we proceeded through Leeds to Manchester. It rained heavily when we arrived at Leeds; but, Mr. Loudon having determined to visit the Botanic Garden, we went there in a most awful thunder-storm, and the whole of the time we were in the garden the rain descended in torrents. We were all wet, and we had no time to change our clothes, as, on our return to the station, we found the last train to Manchester ready to start, and Mr. Loudon was most anxious to proceed thither without delay. When we arrived at Manchester, he was far from well; but notwithstanding, the next morning, though it still rained heavily, he insisted upon going to the Botanic Garden. Here he increased his cold, and when we returned to the inn he was obliged to go to bed. The next morning, however, he would go on to Liverpool; and, though he was so ill there that when we drove to the Botanic Garden he was unable to get out of the coach, and was obliged to send me to look at some plants he wished to have examined, he would sail for Scotland that night. He was very ill during the voyage, and when we landed at Greenock he was in a high fever. He persisted, however, in going by the railway to Paisley, and thence to Crosslee Cottage, where we had promised to spend a few days with our kind friends Mr. and Mrs. Woodhouse. When we arrived there, however, he was obliged instantly to go to bed. A doctor was sent for, who pronounced his disease to be a bilious fever, and for some time his life appeared in great danger.