Taxonomic Description

The following taxonomic description has been derived from Correll and Correll (1972), Fernald (1950), Flora of North America Editorial Committee (2006), Gandhi and Thomas, (1989), Gleason and Cronquist (1963), Lehman, O'Brien, and White (2005), and Radford, Ahles, and Bell (1968).

Seeds

A single seed lacking endosperm is borne in an indehiscent cypsela (achene). Many authors use the term “seed” to designate the seed and the enveloping mature ovary wall.

Seedling Morphology

Seaside goldenrod seeds often germinate in late fall and winter (Orava and Drake, 1997). Seedling recruitment is rare and is limited to open spaces or blowout gaps (Lee, 1993). Leaves, 55 days after germination, are simple, alternately arranged, and gradually tapered and winged to the petiole (Goodwin, 1937). Blades are linear, mostly glabrous, and have a net venation pattern with inconspicuous venation (Goodwin, 1937).

Shoot Morphology

Plants are perennial, 0.4 to 2.5 m tall with short, persistent woody bases (caudices). Stems are 1 to 20, unbranched, erect or ascending, and are usually glabrous. Leaves are simple, alternate, somewhat succulent and glabrous, and the margins are entire. Blades are narrowly ovate to oblanceolate, 10 to 40 cm long and 1 to 7 cm wide, tapered to elongate, and with winged petioles. Blades near the stem apex are numerous and sessile. A basal rosette of leaves is conspicuous.

Inflorescence

The inflorescence consists of a series of heads (capitula). Heads are arranged in paniculate, corymbiform, or recurved-secund branching patterns. Peduncles are glabrous or slightly pubescent and 2 to 3 mm long. Involucres are 3 to 7 mm long, the phyllaries are imbricate, acute, or acuminate, and ciliate on the margins. The heads are radiate. Ray florets are pistillate, yellow, 8 to 17 in number, 5 to 6.5 mm long, and about 0.5 mm wide. Disc florets are bisexual, yellow, 10 to 22 in number, and are 3 to 5.5 mm long, and the lobes are 0.5 to 1.0 mm long. The receptacle is naked.

Fruits

The achenes are obconic, 1 to 3.5 mm long, and strigose pubescent. The pappus is about 3.5 to 5.5 mm long and club shaped.

Variability

Two closely related subspecies of S. sempervirens are recognized in North America, subsp. sempervirens and subsp. mexicana (Semple, 2003). Subspecies sempervirens is a robust plant with lower leaves 2 to 7 cm wide. The involucre is 4 to 7 mm long. The receptacle has 12 to 17 ray florets and 17 to 22 disc florets. Subspecies mexicana is a more slender plant with lower leaves 1 to 3 cm wide. The involucre is 3 to 4.5 mm long. The receptacle contains 7 to 11 ray florets and 10 to 16 disc florets (Figure 1).

Figure 1.

Solidago sempervirens subsp. mexicana in cultivation with monarch butterflies in Brownsville, Texas. Solidago sempervirens subsp. sempervirens (not shown) is morphologically similar at maturity. It has larger heads and broader lower leaves.

Semple et al. (1984) recognized S. sempervirens var. azorica (Hochstetter ex Seubert) H. St. John, which is naturalized or adventive in the Azores. A cultivated variety has been identified as S. sempervirens var. viminea (Ait.) Gray.

Solidago sempervirens is known to hybridize in North America as follows: S. sempervirens × S. rugosa Mill. = Solidago × asperula Desf.), and S. sempervirens × S. canadensis L. = Solidago × erskinei Boivín (Goodwin, 1937; NYFA, 2014). A rare pale yellow color form was described by Harris (1958).

Chromosome Number

Solidago sempervirens has a chromosome number of 2n = 18 with no variation in chromosome numbers across the range of the species and its varieties or subspecies (Goodwin, 1944; Innes and Hermanutz, 1988; Radford, Ahles, and Bell, 1968; Semple et al., 1984).

Substrate Characteristics

Subspecies mexicana typically occurs in low-mineral wet sands and clay soils, and salinity values usually range from 0 to 10 parts per thousand (ppt) in the Upper Gulf of Mexico coastal zone (Stutzenbaker, 1999). In South Carolina, Stalter (1968) reported that this subspecies usually occurs about 1.7 m above mean sea level, and salinity values range from 10 to 40 ppt. He noted that pH values range from 6.0 to 7.5. In southern Florida, Stalter et al. (1999) indicated that soil salinities ranged from 25.4 to 39.9 ppt. On South Padre Island, Texas, Judd, Lonard, and Sides (1977) noted that the depth to the water table in dune depressions where subspecies mexicana occurs is 14 to 56 cm, and 85.9% of sand particles range from 0.18 to 0.25 mm.

Climatic Requirements

Solidago sempervirens, including its two subspecies, occurs over climatic conditions ranging from a tropical to a humid continental (temperate oceanic) climate in Newfoundland where winter-extreme low temperatures may range from −19°C to −32°C (Brown and Cooprider, 2010; Stalter and Lamont, 2006). The species extends from about 18°N latitude in southern Tabasco, Mexico, to about 47°N latitude in southern Newfoundland. In the broad spectrum of climatic zones where this species occurs, plants survive unfavorable conditions through a compact rhizome system and by the production of freeze-resistant achenes. In more temperate, subtropical, and tropical regions plants overwinter as basal rosettes (Cartica and Quinn, 1982; Lee, 1995).

Phenology

Flowering phenophases for both subspecies vary along a N-S geographical gradient (Goodwin, 1944). In New Jersey, Cartica and Quinn (1982) noted that aerial shoots of subspecies sempervirens die in the fall and survive winter conditions in a basal rosette. Fernald (1950), Hill (1986), Lee (1995), and Rhoads and Block (2007) stated that flowering occurs from late July to November. Achenes are mature in late November, and the aerodynamic achenes are dispersed through late April. Peak flowering phenophases correspond to short-day photoperiods (Brauer and Geber, 2002).

Flowering phenophases for subspecies mexicana in warm temperate and subtropical regions are usually later in season. Anderson and Alexander (1985) and Eleuterius and Caldwell (1984) reported flowering in October and November in Florida and Mississippi, respectively, and Lonard and Judd (1989) noted that this subspecies is in flower and fruit throughout the calendar year on South Padre Island, Texas.

Population Dynamics

Natural disturbance is an important factor affecting population dynamics in coastal ecosystems. Seaside goldenrod is referred to as a gap species in blowouts in the dune complex of New Jersey, but it also occurs in immature grasslands and as well as in well-established grassland zones (Lee, 1993). Blowouts and transition zones are sites where seed (achene) rain produces an extensive achene shadow in exposed gaps in the plant cover. Lee (1993) noted that achenes fall to the ground within 10 m outside blowouts. Lower cover values of plants were found in undisturbed sites due to lack of achene dispersal from blowouts.

Achene (Seed) Production and Viability

Achene production, equated with seed production, is extraordinary. Lee (1993) reported a seed rain in blowouts on dunes in New Jersey as 30,700 ±1000 seeds/m². Lee (1995) found that 76% to 98% of subspecies sempervirens seeds germinated from populations sampled in New Jersey. Stalter (1968) and Stalter and Batson (1973) estimated that 67% are viable in populations of subspecies mexicana in South Carolina.

Dispersal

Solidago sempervirens is a widely distributed New World humid continental, temperate, subtropical, and tropical maritime species. Ridley (1930) estimated that the plumed pappus of the achenes allows aerodynamic propagules to travel an estimated 2200 km to oceanic islands. Orava and Drake (1997) also indicated that this species is adapted to oceanic dispersal with subsequent establishment in saline sites.

Minchinton, Simpson, and Bertness (2006) reported that achenes float. Ehrenfeld (1990) stated that subspecies mexicana achenes may be dispersed by ghost crabs (Ocypode albicans) and buried in their burrows. Dispersal by birds is also possible (Lonard and Judd, 1980).

Achene Size and Seed Bank

Achene lengths of subspecies mexicana range from 2.2 to 3.5 mm (Orava and Drake, 1997), and achenes weigh 0.79 mg for subspecies sempervirens (Goodwin, 1937). Lee (1993) reported that the seed bank for subspecies sempervirens is negligible and 0 to 57 seeds/m² were found in sample plots. Most achenes fell from inflorescences between late November and late April on New Jersey coastal dunes (Lee, 1993). Crain, Albertson, and Bertness (2008) found 26 to 38 achenes/m² but found no germinating seeds in the substrate from 0.25-m² plots in Rhode Island brackish marshes.

Germination Ecology and Establishment of Seedlings

Optimal germination of subspecies sempervirens seeds is promoted by freshwater and sunlight and is inhibited by high salinities and shading (Shumway and Bertness, 1992). Complex dormancy mechanisms promote germination in subspecies mexicana after cold stratification and exposure to light (van der Valk, 1974). van der Valk (1974) noted that few seedlings of subspecies mexicana were found on the foredunes of the North Carolina shoreline.

Seeds of subspecies sempervirens exposed to seawater at 30 ppt for 1 to 10 days, washed, and placed in freshwater had 45% to 60% germination success, but seeds exposed to seawater 6 to 10 days germinated more rapidly than seeds exposed for shorter periods and subsequently placed in freshwater (Orava and Drake, 1997). Seeds exposed only to a freshwater treatment germinated at a rate eight times higher than those exposed to seawater (Orava and Drake, 1997).

Vegetative Reproduction

Reproduction in subspecies mexicana in relatively stable sites on North Carolina foredunes is primarily vegetative (van der Valk, 1974). However, Lee (1995) stated that perennial clones of subspecies sempervirens produce numerous compact rosettes on dunes in New Jersey, but vegetative reproduction by compact rhizomes is limited.

Response to Burial

Solidago sempervirens seeds and seedlings do not tolerate excessive burial (Baptista and Shumway, 1998). van der Valk (1974) indicated that seedlings are unable to survive under more than 5 cm of sand. However, high levels of sand deposition are associated with plant vigor in mature clones (Lee, 1995). van der Valk (1974) reported that mature plants survived 56 cm of sand deposition.

Predation

Rabbits and white-tailed deer occasionally browse on juvenile S. sempervirens plants (Sheahan, 2012). Insect predators may cause extensive defoliation and include beetles (Trirhabda borealis) and seed moths (Coleophora triplicus) (Ancheta, Heard, and Lyons, 2010). Ho and Pennings (2013) and Salgado and Pennings (2005) studied the food habits of grasshoppers (Paroxya clavuliger) and aphids (Uroleucon pielou) that feed on S. sempervirens populations in northern and southern latitudes. High-latitude populations of seaside goldenrod have a higher nitrogen content and are more palatable to grasshoppers than plants native to southern latitudes (Salgado and Pennings, 2005). Ho and Pennings (2013) reached a similar conclusion in a feeding study of aphids. High-latitude plants have higher food quality than lower-latitude plants, and insect predators are larger at high altitudes.

Competition and Facilitation

Solidago sempervirens is often regarded as a gap specialist because it occurs in blowouts in dune systems dominated by the perennial grass A. breviligulata (Lee, 1995). Blowouts may bury A. breviligulata clones and this enhances cover and aboveground biomass of S. sempervirens (Lee, 1995). Lee (1995) stated that S. sempervirens, an inferior competitor with A. breviligulata, can coexist in an environment dominated by the superior competitor.

Brauer and Geber (2002) found that seaside goldenrod has low competitive ability in a nonsaline landscape. Physical factors including human-induced changes in soil salinity are correlated with an increase in abundance of seaside goldenrod (Frankowski et al., 1977).

In Massachusetts, Minchinton, Simpson, and Bertness (2006) found that the aggressive dominant, Phragmites australis, excluded S. sempervirens and other species by competitive interactions at the highest elevations of brackish marshes. They noted that the presence of P. australis litter has a negative impact on the colonization of seaside goldenrod and other species.

In a pioneer zone of blowouts and other disturbed sites, the perennial S. sempervirens has negative impacts on smaller annuals (Cheplick and Aliotta, 2009). Weedy grasses including Cenchrus tribuloides, Digitaria sanguinalis, Triplasis purpurea, and the mat-forming forb Mollugo verticillata show significant reductions in vegetative cover in sites adjacent to established populations of S. sempervirens. A taller species, C. tribuloides, is significantly reduced when it occurs in proximity to seaside goldenrod (Cheplick and Aliotta, 2009).

Facilitation or positive interactions occur between the nitrogen-fixing shrub Morella pensylvanica and S. sempervirens and A. breviligulata on the Massachusetts coastline (Shumway, 2000). Solidago sempervirens plants growing under the canopy of M. pensylvanica are more robust, are more likely to flower, have a higher photosynthetic efficiency, and have a higher concentration of nitrates than plants outside the canopy (Shumway, 2000). The environment below the canopy has lower soil temperatures and higher soil nitrogen levels. Therefore, growth, reproduction, and recruitment of S. sempervirens are facilitated.

In a study of the interactions of competition and physical environmental factors, Hacker and Bertness (1999) found that physical conditions in the lower middle salt marshes of Rhode Island are so harsh as to eliminate interactions of S. sempervirens with other species. All S. sempervirens clones transplanted into this habitat died with or without the co-occurrence of the dominant species Juncus gerardii. In the upper middle marsh J. gerardii facilitates the growth and reproduction of S. sempervirens. Therefore, S. sempervirens is capable of competitive tolerance in the higher elevations of a salt marsh (Hacker and Bertness, 1999).

Wildlife Values

Seaside goldenrod has several wildlife values. It serves as a source of nectar for bees and migrating butterflies (Stutzenbaker, 1999), and has been recommended for cultivation in butterfly gardens (Richardson and King, 2011).

Plant communities where S. sempervirens occurs are important concealment and nesting sites for birds. Craik and Titman (2009) found that red-breasted mergansers (Mergus serrator) and the common tern (Sterna hirundo) nest in this habitat in New Brunswick, Canada. In New York, Safina and Burger (1983) found that Solidago sempervirens and other species provide nesting habitat for willets (Catoptrophorus semipalmatus), killdeer (Chradrus vociferous), piping plovers (C. melodus), black skimmers (Rynchops niger), common terns, and roseate terns (Sterna dougalli).

Medicinal Uses

Folk literature has some reports of medicinal uses for various species of Solidago, but none is listed for S. sempervirens and its subspecies. All species of Solidago are insect pollinated. Pollen of the genus Solidago does not cause respiratory allergies.

Potential Biological Control Agents

Cheplick and Aliota (2009) found that densities and plant heights of weedy, annual pioneer grasses shaded by S. sempervirens were significantly reduced. Triplasis purpurea, Cenchrus tribuloides, and D. sanguinalis were affected, but the annual, prostrate, broad-leaved dicot, Mollugo verticillata, was not. A diterpene, sempervirenic acid, has been isolated from S. sempervirens (Purushothaman et al., 1983). Terpenes are unsaturated hydrocarbons in most oleoresins and essential oils in plants. Exudates of sempervirenic acid from shoots, roots, and rhizomes of S. sempervirens clones may be involved in allelopathic effects on low-growing annual grasses. However, this hypothesis has not been tested.