Phenology of some Iridaceae

The graph below plots the date on which the first flower has opened on each of six Moraea species in my garden over 18 years. The site is the Adelaide suburb of Warradale, close to sea level at 35° S latitude. All six species are summer dormant, spring flowering corms from South Africa where they evolved in Mediterranean-type climates similar to Adelaide.

This kind of “experiment” is rather in the tradition of Gilbert White, or those other English country parsons who competed to report the first cuckoo of spring in letters to The Times. Among its many limitations,

• The date of first anthesis may not be the best way to characterise flowering period; a measure of peak flowering may be more meaningful
• Microclimate was not controllable, although all plants were grown in the open in the same garden
• Each species was represented by only one genotype
• There are gaps in the records when a species either failed to flower or was not observed in a particular year

However, the graph seems to justify some comments:

1. The sequence of flowering shows some constancy, with a ‘late’ group of M. bellendenii, M. ochroleuca and M. setifolia always flowering after the ‘early’ M. aristata, M. flaccida and M. vegeta.
2. Flowering time did not change in the same direction or to the same extent for all species in each year. Each species has its own response to the weather that it experiences. For example, M. aristata and M. vegeta moved in contrary motion much of the time, but in near unison from 2007 to 2010 (yes, that’s a musical metaphor!).
3. There may have been a general trend to earlier flowering in the “global warming” period of the late 1990s. Subsequently, flowering times may reflect the southern oscillation with late flowering in the notably cool, wet years of 2010 and 2011.

The mechanism controlling flowering has not been investigated in these Moraea species, but experience suggests it is not determined by photoperiod but by the accumulation of sufficient shoot biomass to support flowering and fruiting.

A second graph gives the corresponding results for seven species of Watsonia with similar phenology to the Moraea.

Again, the sequence of flowering shows some constancy, with W. aletroides consistently the first to flower and W. angusta or W. marginata usually the last. Again, each species reacted in its own way to seasonal conditions. Again, some of them seem to show the effect of a run of hot dry years in the late 1990s but more complex responses to the alternation of La Nina and El Nino periods. The response of W. laccata was especially variable, possibly as (unlike the other six), this species was represented by a population of mixed genotypes from seed.

The combined effects of temperature, insolation and rainfall may be complex, and biomass accumulation may not be easily modelled by the day-degrees method that is useful in higher latitudes where plant growth is closely correlated with temperature. The effects of rainfall and temperature may vary greatly according to the part of the growing cycle that they impact, and cannot be understood by integrating them across the whole growing season.

A cool, overcast winter may delay or inhibit flowering in these irids by slowing their biomass accumulation. As commercial Gladiolus growers have found, flowering fails if winter light intensities are too low. Drought in the period of vegetative growth has the same result. The first elongation of the flowering stem in Moraea flaccida and Watsonia meriana has been shown to coincide with the exhaustion of the old corm from which the plant has grown and the beginning of resource allocation to the formation of the next season’s corm. This is the most vulnerable point in the annual cycle of these irids. Once flowering has been initiated, even a few days of hot dry weather in spring can goad the plants into completing their flowering rapidly by producing fewer and smaller flowers.

(Please click on the graphs if you want to see them full size)