Mild temperatures may hurt plants that prefer extremes

By Tony Tomeo

The very mild weather this past summer which has made gardening so pleasant has been less than ideal for many species of plants which prefer heat. Likewise, the mild winter weather will likely be detrimental to those species which require cold weather for sufficient dormancy. Many plant species require both warm summers and cool winters to maintain healthy growth and dormancy cycles.

If deprived of necessary weather conditions, these species respond by adjusting their schedules. For example, my neighbor's peach tree lacked the energy necessary to outgrow peach leaf curl due to the lack of warm weather. It later went into a very slow dormancy because of the lack of cold temperatures. If the weather remains mild through winter, the tree will likely bloom early and less profusely, losing its flowers or juvenile fruit in the cool, rainy weather of late winter or early spring. It is as if the peach tree has had a very stressful day at work, was unable to sleep that night and needed to go back to work early the following morning.

Of course, we can not correct the altered schedules of the plants in our gardens any easier than we can change the weather. However, if we know what to expect as a result of the mild seasons, the unusual behavior of our plants will not be so surprising or be attributed to improper cultural practices.

Although the mild winter is difficult for many plants, is great for working in the garden. Remember that there are many cool season vegetables that can be planted this time of year, as well as flowering annuals, such as Iceland poppy, nemesia, schizanthus, pansy, viola and various primroses. Because most deciduous fruit trees will come out of dormancy early, pruning may be done shortly after all the leaves have fallen off. If you prefer to wait, be certain to get the pruning done before the buds begin to swell. Pruning performed after dormancy has ended can be very damaging.

Grafting is also performed in winter, when plants are least active. In the nursery production industry, grafting is performed for various reasons. Many cultivars (clones) that are difficult to grow from cuttings may be grafted onto understocks (the plants providing the root system), which are more easily grown from cuttings or seed. I often graft Camellia reticulata onto Camellia sasanqua understock because C. reticulata does not grow well from cuttings, but C. sasanqua does. Many plants, such as dwarf citrus, are grafted onto understocks which limit the ultimate size of the mature plants.

Grafting also can be employed to provide a root system that is either better behaved or better suited to local soil conditions than the root system normally associated with a particular species, such as English walnut grafted onto the more durable roots of black walnut. In home gardening, grafting can be useful for many other reasons. Rather than replacing an undesirable fruit tree, a desirable variety may be grafted onto it to utilize the existing root system. This may be done on the trunk after the tree has been cut down, or by adding the graft to the canopy of the tree with the intention of phasing out the older branches. If a fruit tree provides more fruit than can be used, various clones may be added to replace much of the original tree and provide smaller amounts of fruit at various times through the season. The cumulative production is usually comparable to that of the original tree.

Grafting technique requires more explanation than I can write about. Fortunately, many publications describe the procedure for those who are interested. Sunset's Western Garden Book is an excellent source of information.

Compatibility between scion (stem which is grafted) and rootstock is a very important concern, which is somehow not mentioned in most descriptions of grafting. It is therefore important to remember that almost all grafts are between plants of the same genus. For example, apples may be grafted to other apples; oranges may be grafted to other citrus; but you cannot be talking oranges and apples! Incidentally, the genus Prunus includes apricots, nectarines, peaches and plums among other things, many of which may be grafted together as a tree producing various fruit.

Flower of the Week: Sweet Pea

I was earlier advised to write about the century plant, Agave americana, and its current insect problems, but I thought sweet pea, Lathyrus odoratus, would be more interesting this time year. (It seems that century plants cannot be planted in groups of more than 20. The 21st-century plant always has problems with the Y2K bug.)

Sweet peas are actually a spring annual, but perform very well if planted in winter. Spring planting is often too late because warm weather starts early, ending their season. Bush type and summer flowering sweet peas are more heat tolerant and can grow into summer, but usually either roast in dry weather or succumb to mildew in humid weather.

Early flowering plants are slower, but grow over a longer season, providing their distinctive flowers in shades of blue, purple, rose, scarlet, pink, lavender and white. The spindly vines cling to light trellises or hedges (which are not pruned in winter), and may be as high as six feet. They perform best when seeded directly into the garden. Although sweet peas do not fill flower beds like most flowering annuals, they are a nice addition to gardens that lack color in cool weather.

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Horticulturist Tony Tomeo can be reached at 408.358.2574.

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