The recent spell of 100-degree days is one good reason we have trouble growing plants that like cool nights and sunny but not hot days, and why perennial borders do not flourish here.
Even when the plants grow well enough, our brilliant sun and rigorous heat causes things to grow much faster than in Britain. Here almost any plant gathers strength early, blooms in a great magnificent burst, then rests at length.
Thus a clematis that blooms "from June to October" in England is likely to bloom in mid-May here, and be over by June, with a little spate of flowers again in September. I once planted 'Lady Betty Balfour,' a purple clematis with light stamens, mainly because it bloomed the end of the summer. But with me it was in bloom by June 10, and over by July.
Again, I found that potentillas did not do much for the garden here. These twiggy little bushes bloomed heavily in spring, then did nothing until the following spring. In Minnesota I have seen the same shrublets blooming steadily through the summer, and they do the same in England, where they are valued occupants of almost any garden.
But I do not much care if a small shrub blooms its head off in April or May -- there are lilacs and mock oranges and peonies, irises and roses then. The value of the potentilla is properly that it blooms moderately for week after week, and since it will not do that in my garden, I see no reason to grow it.
Again, look at roses. It depends a little on the season, whether spring is early or late, but our earliest roses come in late April and the main flush is in May, lasting into June. But in England, if roses are pruned as hybrid teas usually are, they do not even begin until July or quite late in June. One year in England, at the national collection of roses at St. Albans, there was scarcely a rose to be seen early in July -- that year it was mid-July before they made any real show. And this year, early in July, there were plenty of roses there, but they were the first roses, and some varieties had not yet opened.
Naturally, if a rose starts blooming two months later than with us, it will bloom fairly steadily all summer. When Americans visit English gardens (and the Scotch gardens are even later) we are overcome with admiration for the mass of flowers in July, forgetting that these same gardens did not look like much in May.
Often I think we should have done much more with those flowers that relish (not merely endure) our light and heat. Morning glories, moon vines, cypress vines, angel trumpets, large periwinkles, verbenas, marvels of Peru, crinums, yuccas -- these are the kinds of things that flourish in our Julys and Augusts.
Sometimes a plant like the yucca will be taken in hand by some enthusiast, and many varieties will be raised, but these do not get into general circulation and are soon lost. They are stately plants, of course, and if some effort were made we could have them in pink as well as white, and in considerable variety of habit. Similarly the hesperaloes could be tamed for gardens.
The climbing solanums (closely related to the potato) in blue and white are rarely if ever seen in our gardens because they are not hardy as a rule. But if taken in hand, it would surely be found that they could be selected for hardiness, if given a mulch in the winter and grown against a warm wall, and their clusters of soft blue flowers would be welcome.
Erythrinas, sometimes called cry-baby trees, are widely seen along the Gulf of Mexico, but the commonest one, the coral bush, is perfectly hardy here if given a sunny spot, mulched in the winter, and given the usual common sense applied to plants from warmer climates than ours. Although fully tropical and subtropical, the plant makes a tough root and while the top is killed to the ground every winter, new growth arises in the spring and the huge clusters of waxy red pea-shaped flowers are stunning. I knew one that had grown for many years in Tennessee, where in the same climatic zone as Washington, it had endured plenty of zero winters. The owner told me it had been propagated from one on the grave of Robert Louis Stevenson in Samoa, a thing I mention to show that some tropical plants bear cold when dormant.
Daturas, or angel trumpets, are a somewhat confused (in gardens) group of plants that include the jimson weed, but also beautiful shrubby perennials (often treated as annuals) with white trumpet flowers a foot long in our hottest weather. In moderate winters, such as the one this year, they sprout from the old stems near ground level. In severe winters they are killed outright but are easily grown from seed like zinnias.
A lovely morning glory, from Brazil, I believe, is variously called Pharbitis learii or Ipomoea learii. It is perennial but will die in winter if not protected. In the past I have pulled it through several subzero winters by the simple expedient of giving it a warm sheltered spot to begin with and, if I thought of it, a mulch of honeysuckle branches about Thanksgiving, removing them in April. The thing about it is that its flowers are borne on six-inch stems in clusters, opening in succession. It is electric blue, a more exciting color than 'Heavenly Blue,' for instance, though it begins to turn magenta about breakfast time and is no good except early in the morning. But for gardeners who get up early in the summer it is a great treasure. Yet I have never seen or heard of it in gardens here, except occasionally in greenhouses. It should be planted outdoors from pot plants in May and given plenty of water and general encouragement until August, when it can be allowed to dry off a bit and harden up for winter.
Most gardeners have had the experience of dahlias and gladiolus surviving the winter without protection. This is not the way to grow them -- they get congested if not lifted and stored indoors over winter -- yet they will go on for some years if given a sheltered sunny spot. And many of the miniature gladiolus can be planted six inches or so deep and left to fend for themselves, making great fat clumps and useful for cutting. Unfortunately for me, I am not wild about either, though I acknowledge their considerable beauty.
How often you see a south-facing wall with a narrow strip of earth and nothing much in it except Bermuda grass, an expiring tomato and a clump of marigolds. But what a fine place for crinums. Many of them are much hardier that the gardener thinks. The trick is to plant them from as large bulbs as can be obtained, in May. It is possible to lose them if planted outdoors in early April -- they sprout leaves that can be damaged by cool snaps then, and may rot in the heavy rains of the season, but once it gets nice and warm in May they go right ahead. It may take them two years, or even three, to settle down and start blooming well, but then they produce waist-high thick stalks with clusters of pink or white flowers like lilies or amaryllis, going on for weeks. When they make fat clumps they are almost always in bloom.
The night jasmine (Cestrum nocturnum) is a half-weedy shrub valuable only for its panicles of intensely perfumed inconspicuous flowers late in summer. Usually it is its best around Labor Day. It needs winter protection, preferably a south wall, though it is usually brought indoors, or grown from cuttings in the fall to grow on indoors and to be set out in May. But it is best in a large plant several years old, when it will reach chest height and scent the whole house if the windows are open.
We should give more attention to things like these -- we already do well with petunias and zinnias, fine inhabitants of our summer gardens -- but should consider the many other plants that think (as I do) our summers are splendid.