SAGUARO Ecology of the Saguaro: II NPS Scientific Monograph No. 8

Dr. Parry was informed that the flowers were produced in May and June, from the summit of stem and branches; they are said to be white, with a red centre, and three inches in diameter. The fruit matures in August, and is set with small spines: it is obovate, one and a half inches in diameter, red, pulpy, of sweet taste.—George Engelmann, "Notes on the Cereus giganteus of southeastern California, and some other Californian Cactaceae," 1852:337.

Phenology

The saguaro blooms and produces a crop of succulent fruits during the driest period of the year—the hot, often rainless months of late spring and early summer. In the absence of other moisture, fruit development is insured by the reservoir of moisture stored within the succulent stem. Thus, each year, the saguaro produces a large crop of seeds that mature just prior to the start of germinating summer rains.

The chronology of saguaro flower and fruit production has been described by Engelmann (1854) and subsequently by other observers including Toumey (1897), MacDougal and Spalding (1910), Shreve (1929), and Thackery and Leding (1929). In addition, numerous references concerning historical observations on the timing of saguaro fruit harvest by aboriginal peoples are noted by Castetter and Bell (1937). Johnson (1924) reported on the effects of solar radiation and internal stem temperatures on the development of blooms.

The timing of saguaro flower and fruit production appears to be controlled primarily by the arrival of warm spring temperatures coupled to increasing day length (Johnson 1924). This is reflected in the difference in the date of the appearance of the first buds, blooms, and the beginning, peak, and decline of fruit maturation in different portions of the plant's geographic range in Arizona and Sonora.

The initiation and development of buds, flowers, and fruits occurs on progressively later dates from south to north, and from lowest to highest elevations. Moreover, in the Tucson area there is a lag of 1-2 weeks between the first appearance of buds and open flowers at the Saguaro National Monument west of Tucson and their development at the original higher elevated monument 30 miles (48.3 km) eastward. The difference in flowering time (also dates for budding and fruiting) between these saguaro populations is due to relative differences in temperature that result from differences in elevation, topographic association, cold air drainage, and slope exposure (Table 1).

Flowering date and the time for saguaro fruit maturation are highly variable from year to year depending upon the temperatures occurring during the late winter and spring of the particular year and the genetically controlled variability within the population.

In the vicinity of Tucson, the first flower buds ordinarily appear on most individuals sometime during the last 15 days of April. The blooming peak ordinarily occurs during the last week of May or the first week of June. A few individuals bear their first ripe fruits at that time. Thereafter, the number of ripe fruits increases rapidly, reaching a peak that occurs from the last week of June into the second week of July according to year. The remaining fruits ripen and the number remaining on the plants falls abruptly during the following 10 days in any year (Fig. 13A).

Fig. 13A. Approximately 100 ripening fruits on a 4.5-m (14.8-ft) saguaro in mid-June. Fruits develop during the driest period of the year—the hot and usually rainless months of late spring and early summer. The peak of fruit ripening in the Tucson vicinity occurs from the last week of June into the second week of July, depending upon the early or late arrival of warm spring temperatures in March-April. Photographed 17 June 1974.

A few individual saguaros produce blooms and mature fruits in late summer, fall, and even during winter months. On one plant, ripening fruits were observed on 6 January 1969. Other genetically divergent individuals have been observed to bloom in December, and in September and October. Generally, such blooming produces only a few fruits (or none), and these are usually consumed by birds as fast as they ripen. It is unlikely that any germination results from such off-season fruit production, and none has been observed, inasmuch as germination is associated with the summer warm-season monsoon.

Early or late arrival of warm spring temperatures (March into May) can produce substantial year-to-year variation in the timing of bud, flower, and fruit development. Similarly, because of warmer temperatures there, the peak of fruit maturation regularly occurs approximately 5-7 days earlier in habitats of the western (Tucson Mountain) section of the monument (elevations upward from 616 m [2021 ft]) than in the comparatively cooler habitats of the eastern (Rincon Mountain) section of the monument (elevations upward from 823 m [2700 ft]; Fig. 13, Table 1). A similar lag of a week to 10 days in any given year occurs from the edge of the Gulf of California (e.g., in the vicinity of Bahia Kino) inland to the mountains in the vicinity of Hermosillo, Sonora, a distance of 80 km (50 miles) and difference of 700 m (2297 ft) elevation.

At Sacaton, Arizona, Thackery and Leding (1929) observed that the period for development and ripening of saguaro fruits averaged 30 days for flowers that appeared in June. At Tucson, McGregor et al. (1962) found that the average time for fruit development from flowering to maturity was 37 days (range 31-45 days). They also observed that fruits from late-blooming flowers matured at a faster rate (the low end of the range) than those which opened at an earlier date.

During years when flowering is delayed by cold spring temperatures, we have observed that there occurs a similar speed-up in fruit development. During occasional years when low temperatures delay spring blooming by as much as a full month, the subsequent delay in fruit development may be only 7-10 days due to process "catch-up" toward the end of the reproductive season. Such a 1-2 week delay in seed drop coupled with an earlier-than-usual arrival of good summer rains have occasionally produced a bumper crop of saguaro seedlings.

The flowers of the saguaro open during the early nighttime hours and remain open until afternoon of the following day (see Peebles and Parker 1941). We have found that saguaro flowers and those of other large columnar species stay open longer on cool days and on days earlier (cooler) in the spring flowering season.

Natural pollination, which is accomplished by animals, occurs primarily during nighttime and early morning hours. Cross-pollination, essential for fertilization and fruit development, is effected by a variety of flying animals (insects, birds, and bats) that feed upon the pollen and copious nectar of the open flowers (MacDougal 1905; Alcorn et al. 1959, 1961; McGregor et al. 1959, 1962).