Found throughout Britain, becoming absent on high ground in Scotland (3). It also occurs throughout much of the rest of Europe, as well as North Africa, Western Asia and the Caucasus region of south west Russia (2). Yellow iris has become widely naturalised outside of its original range, as it is so popular in gardens (3).
Seedling establishment and plant growth: Pale-yellow iris generally establishes in areas that are moist but not waterlogged. In many cases, pale-yellow iris establishes on the edges of water features (see Site characteristics), as water-dispersed seeds are often deposited along the high water mark (reviews by [35,78]). On Theodore Roosevelt Island in the Potomac River, pale-yellow iris established on trash, river debris, and tree roots that were above the general water level of the marsh . As a young plant, pale-yellow iris invests heavily in developing a root system to adapt to fluctuating water levels .
Water plays a key role in pale-yellow iris growth. While pale-yellow iris needs moisture, laboratory experiments show that inundated conditions reduce seedling growth [11,43]. Seedlings may recover after inundation . On Theodore Roosevelt Island in the Potomac River near Washington, DC, the length of time that pale-yellow iris was inundated by water was the factor most limiting to its growth; areas experiencing short inundation exhibited greater growth than areas with long inundation . In contrast, in Montana, pale-yellow iris plants growing in 2 to 3 feet (1 m) of water were larger and more "vigorous" than plants that were not inundated . Pale-yellow iris occurred in areas that were flooded for as long as 6 months in its native range [53,86]. One author reports that in England, pale-yellow iris seeds germinated and seedlings grew well in marshes burned in late summer and flooded over winter (review by ).
Studies in Poland show most mortality of pale-yellow iris seedlings occurs in the first 2 months after germination, most likely due to desiccation. Heavy seedling mortality also occurs the first winter due to freezing surface water. In Poland, between 28% and 72% of seedlings survive their 1st year; only 3% to 6% grow to a size at which the rhizomes fragment (review by ) (see Vegetative regeneration).
Light conditions impact the growth pattern of pale-yellow iris. In its native range, pale-yellow iris tends to flower more in open habitats, which leads to a more clumped distribution as rhizome branching is associated with flowering. In wooded or shaded habitats, less flowering occurs and plants tend to spread linearly. Shaded plants tend to have fewer and longer leaves than plants in open areas (review by ). One study suggests that low light may limit seedling establishment but not growth of mature pale-yellow iris plants .
On the fringes of saltmarshes in Ireland, shore level influenced pale-yellow iris growth. On high saltmarsh sites plants had more and longer leaves and high rhizome terminal bud survival compared to low saltmarsh sites .
This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [15,21,22,25,28,29,51,56,62,69,88,92,97]).
Aboveground description: The few to several leaves of pale-yellow iris are stiff and erect , linear, and 10 to 35 inches (25-90 cm) long . Single or multiple flowering stems are 20 to 39 inches (50-100 cm) tall, usually shorter than or equaling leaves . Plants take 3 years to mature before flowering (review by ). Flowers are bright yellow or cream-colored and 3 to 4 inches (7-9 cm) wide . Pale-yellow iris fruits are 6-angled capsules, 2 to 4 inches (5-9 cm) long . The dark brown, smooth, disk-like seeds are closely packed into 3 rows within the capsule (review by ). Seeds have a hard seed coat beneath which there is a gas space, allowing seeds to float in water .
Belowground description: Pale-yellow iris has rhizomes that are 0.4 to 2 inches (1-4 cm) in diameter (reviews by [73,90]). Pale-yellow iris invests heavily in root development, particularly as a young plant, allowing it to establish in habitats with fluctuating water levels . Roots are usually 4 to 8 inches (10-20 cm) long but can be up to 12 inches (30 cm) long (review by ).
Pale-yellow iris germination is best in moist [11,77] but not waterlogged [42,77] conditions. One review suggests that pale-yellow iris germination may be more dependent on temperature than light . In the field, pale-yellow iris germination in its native range is said to be poor due to fat-like substances present on the inner seed coat. Seedlings were rare in most habitats (review by ). However, one author states that a "large number" of pale-yellow iris seeds in Great Britain germinate and put out roots . In Montana, one author suggests that pale-yellow iris germination rates were high based on the high number of seedlings observed in the field and from observations of field-collected seed .
In the laboratory, only 25% of fresh-collected seeds from the Netherlands germinated in drained soil in 6 weeks . Germination rates for nonscarified seeds in Great Britain varied from 48% after 15 days to 40% after 12 months (review by ). Fresh seed collected from established plants in northern Florida exhibited a germination rate of 62% (Jacono and Ramsey unpublished data cited in ). Fall-collected seed from the Theodore Roosevelt Island in the Potomac River had a germination rate of 33% after 97 days of inundated conditions and 90 days of moist conditions .
Scarification improves pale-yellow iris germination ([11,26], review by ). In a laboratory study in northern England, scarified pale-yellow iris seeds germinated at a significantly higher rate than those that were not scarified (70% versus 48%) (P<0.01) . In laboratory studies in the United States, pale-yellow iris seeds that had their "caps" removed and were placed in water experienced much higher germination; in 30 days, 97% of seeds with caps removed had germinated, while no seeds with intact caps germinated in this time period .
It is not clear how long pale-yellow iris seeds persist in the soil seed bank. Pale-yellow iris was abundant in a wet meadow in northeastern France but was absent from the soil seed bank . Though present at 84% of the quaking fen locations sampled in the Netherlands, pale-yellow iris germinated from only 25% of the seed bank samples. Similarly, though present at 50% of floating forest locations sampled, pale-yellow iris germinated from only 14% of the seed bank samples . Pale-yellow iris seedlings emerged from soil samples taken from tidal freshwater marshes along the Delaware River in New Jersey .