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What is a Phlox?

Horticultural History

Accounts of Species and Cultivars


The general conclusion we can make about the development of phlox cultivars during the time the genus has been in cultivation is that there has been surprisingly little in the way of planned breeding toward specific goals. This has been done with P. paniculata and perhaps to some degree with P. subulata, but as for the other species there has been simply selection from wild populations, garden collected seed, or chance seedlings.

The P. paniculata breeders have had clear goals in mind. Symons-Jeune was interested in breeding plants for the landscape border, and had a clear picture in his mind of plants with large flowers ("pips") of clear color and well-filled and -proportioned panicles ("trusses") that could be appreciated from some distance. Alan Bloom originally grew plants for the cut flower market and would have had even more exacting standards of flower and panicle quality. The modern Dutch breeders have size specifications in mind, and American breeders look for mildew resistance combined with traditional size and flower appearance. In fact many of the new American cultivars are simply found seedlings that do not differ much from wild forms. It is as if  this selection is going on in complete disregard of any previous breeding work that has been done.  

These goals are just part of what should be considered in a breeding program. The sidebar at right lists all of the traits that I can think of that are important to phlox as a whole as garden plants along with the direction of their desirable expression. Some of the traits are concerned with survival in the garden and others, like "not sticky glandular," with being gardener-friendly. But most are related to appearance – shape or color. This sort of value system assumes that there are aesthetic standards that can be applied to plant appearance. This seems to be a universally accepted assumption, and, presumably the mind judges plants using some of the same criteria as for works of art. These standards do not apply to plants that are bizarre mutations and marketed as novelties.

Sources of Desirable Traits

Obviously the normal traits that are found in wild plants are those that have evolved for the greatest fitness under natural conditions. Growth habit is an adaptation to the site; flower structure and color are adaptations for pollination. What humans find desirable in plants may have little to do with fitness in nature. The plants that we refer to as “improved” seem better to us only because their growth habits and proportions or their petal coloration are more pleasing.

We can breed plants that have the degree of expression we want of series of desirable traits by 1) finding unusual individuals with desirable single traits and bringing them into our stock, 2) by recombining groups of traits that tend toward the desirable, or 3) by hybridizing with other subspecies or species that have these traits. In the case of 1 and 2 we are working within the normal boundaries of variation in the subspecies or species, and the results of the crosses that we make will be more or less predictable since the gene combinations that result in trait expression are similar. With 3 we are moving into unpredictable territory, since different genes may be used in wild populations to produce similar phenotypes. When these genes are combined in hybrids the phenotypes produced may be outside the normal range of either parental type. For example, crossing two species of short stature may produce a set of F1 hybrids that have a wide range of heights from shorter to taller than the range of heights of either parental species. Other traits such as flower color may be affected in novel ways.

The range of cultivar forms in P. paniculata gives some indication of what sort of variation is possible in garden phlox. In addition to the forms and flower colors of P. paniculata there are also different traits from other species that could be brought together. Even when direct hybridization between certain species or cultivars does not work, it may be possible to bring about recombination through other more compatible plants.


Varying trait

    Desirable direction
Height X width (non-moss)
Bushiness (density)
Degree of floral cover
Rate of spread
Bloom time and duration
Ease of cultivation
Disease and pest resistance

stocky but not squat
stays erect, not floppy
medium to dense
approaching 100%
slowly spreading clump
early and long
very hardy
very easy
Leaf size
Leaf shape

dark green
not sticky glandular
in proportion
pointed, about 1:4 or 1:5 W:L
Basal angle
Internode length

not sticky glandular
approaching 90 degrees
in proportion
Flower panicle
Number of flowers

in proportion to overall
20-30 flowers open at once
flowers touching, not crowded
Corolla plane
Petal lobe width
Tube length

large, up to >30mm diameter
clear, not muddy
nearly flat
wide to make full corolla
in proportion
sweet, pronounced

 [Introduction] [What is a Phlox?] [Horticultural History] [Accounts of Species and Cultivars] [References