The principles governing climbers for house walls apply also to garden walls, but the garden wall gives us some splendid opportunities in addition. I used to dread the idea of high garden walls, but I believe we must borrow this custom from England. The seven reasons therefor have been stated at the end of Chapter IX. Only one will be developed here. The shelter of the garden wall enables Englishmen to grow many of the greatest treasures of subtropical regions including shrubs trained like climbers. Among such treasures are the true myrtle, the poet's laurel, Magnolia grandiflora, the large-leaved Algerian ivy, the pomegranate, laurustinus, and camellia — an intoxicating array of evergreens when contrasted with the solitary pair that are hardy with us — ivy and euonymus.
Of course, the Bostonians could never grow these evergreen climbers outdoors even behind high walls, and even at Philadelphia these plants might not be both hardy and evergreen. But New York and Philadelphia would doubtless pick up other treasures they could enjoy to the full in no other way, e. g., the evergreen magnolias, and a large-leaved ivy from China, and the cherry laurel from Schipka Pass. Moreover, it is possible that, on their north and west walls, Japanese honeysuckle would hold their leaves all winter; also the memorial rose and the scarlet honeysuckle. And by the same device Baltimore and Washington might be able to grow some of the plants that are evergreen at Augusta, such as the Cherokee and Macartney rose, the cross vine, the Carolina jasmine, the Confederate jasmine, Irish and Algerian ivy, the dwarf fig, the Constance Elliott passion flower and several bignonias. The accuracy of these examples cannot be vouched for. The list is merely suggestive.
Among the famous deciduous plants from warmer climes which the English (in the extreme South) can grow on walls are the Ceanothus of California, the Lapageria of Chile, and the Clianthus of Australia. But I doubt if any climbers from those countries can ever be a great success east of the Rockies. For a plant that has never encountered zero weather in its own home can never do so elsewhere. In England the great problem is to get enough sunshine in summer to ripen the wood sufficiently to withstand the mild English winter where zero weather is unknown. South walls help them solve that problem. But in America no walls can keep roots from freezing in zero weather. Nevertheless, I have faith to believe that walls would help us grow in any given locality many fine plants that belong to the South. For example, the winter sweet and winter jasmine will open their fragrant flowers in March or earlier in New York and Philadelphia, and Forsythia suspensa is very beautiful when planted above a wall and allowed to hang down. Philadelphia can grow the white jasmine of the poets on garden walls. And there are three special beauties in England which I hope some of my readers will try.
The first is the tamarix, with feathery gray foliage and feathery pink bloom, which never looks well as a shrub (except when kept low as on plate 105), but is unspeakably lovely when trained against a wall, as at Trinity College, Cambridge.
The second is Xanthoceras sorbifolia, which has foliage like a mountain ash and white flowers marked with pinkish purple. I saw a specimen about twenty-five feet high on the house of Mr. R. Irwin Lynch, the scholarly and companionable curator of the Cambridge Botanical Garden.
The third is lemon verbena, which sometimes covers a space twelve feet square on English house walls. It is the only wall plant with fragrant foliage that I know and it must be pleasant to have every breeze from April to September bear the odour of lemon into the house. I wish my friends in Baltimore, Cincinnati, and St. Louis would try this.
A moment ago I said that garden walls may shelter more precious climbers than house walls. They have another advantage. Even if you desire a perfectly smooth brick wall there is a good way of training climbers that cannot climb alone. For, when the walls are building, you may lay in the mortar at regular intervals iron hooks or something that will not be as objectionable on garden walls as on house walls.