Invasiveness Assessment - Holly (Ilex aquifolium) in Victoria

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Plant invasiveness is determined by evaluating a plant’s biological and ecological characteristics against criteria that encompass establishment requirements, growth rate and competitive ability, methods of reproduction, and dispersal mechanisms.

Each characteristic, or criterion, is assessed against a list of intensity ratings. Depending upon information found, a rating of Low, Medium Low, Medium High or High is assigned to that criterion. Where no data is available to answer a criterion, a rating of medium (M) is applied. A description of the invasiveness criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the invasiveness of Holly.

A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:

Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1 MB)
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Common Name: Holly
Scientific name: Ilex aquifolium

Question	Comments	Rating	Confidence
Establishment
Germination requirements?	In Spain germination is reported to occur from Spring to Autumn, with the highest proportion germinating in late spring (Arrieta & Suárez 2005). Therefore the species germinates under seasonal conditions.	MH	H
Establishment requirements?	Can establish in the dense shade underneath mature holly trees, which is reported to be 1% of sunlight, however survival rates are low (Arrieta & Suárez 2005; Peterken 1966). Reported to invade the edges of cool temperate rainforest (Muyt 2001). Survival of holly seedling through their first year is reported to be effected by the summer drought period and grazing (Arrieta & Suárez 2006). The species can therefore establish under a moderate canopy but does require other additional factors such adequate moisture through the summer period.	MH	H
How much disturbance is required?	Reported to invade wet sclerophyll forest and riparian vegetation (Carr, Yugovic & Robinson 1992).	MH	MH
Growth/Competitive
Life form?	Other; Holly is a “dioecious broad-leaved evergreen tree” (Obeso 1998).	L	H
Allelopathic properties?	No reported evidence of allelopathy in Holly.	L	M
Tolerates herb pressure?	Holly is noted as being browsed by ungulates including cattle, goats and deer. A plant exposed to heavy grazing form establishment can persist as a low gnarled mounded shrub and the more a plant is browsed the spinier the leaves of the regrowth become (Obeso 1997; Perrin, Kelly & Mitchell 2006). The species is affected by a leaf-miner (Phytomyza ilicis) in Europe (Eber 2001). A study where individual branches were artificially girdled or defoliated showed little impact on fruit production and therefore sexual reproduction (Obeso 1998). Therefore the species can persist under gazing and is capable of flowering and producing seed under moderate grazing.	MH	H
Normal growth rate?	The species is reported to be slow growing laying down very hard wood also seedling growth is reportedly slow, taking five or more years to reach maturity (Blood 2001). Therefore the growth rate of holly is considered to be less than many other tree species.	ML	MH
Stress tolerance to frost, drought, w/logg, sal. etc?	Tolerance to frost varies with time of the year from -9ºC in summer to -24ºC in mid-winter (Rütten & Santarius 1988). Tolerates frost to -15ºC for short periods (Blood 2001). Prolonged severe frosts can damage trees, the species is reported to be limited to areas in Europe where the mean January temperature is above -4.6ºC (Peterken & Loyd 1967). Therefore the species is considered to be tolerant of frost. Drought can kill seedlings within their first year (Blood 2001). Holly leaves are sclerophyllous which can be an advantage under moderate drought conditions (Salleo, Nardini & Lo Gullo 1997). Therefore while mature plants may be able to persist through a period of drought, the species is unlikely to establish in a drought prone area and therefore is not considered tolerant to drought. The species shows some tolerance of heavy metals (Samecka-Cymerman & Kempers 1999). Fire can kill the stem, but the plant can then resprout from the base (Peterken & Loyd 1967). Reported to be reduced after forest fires (Tinner et al 1999). Therefore the species is considered to have some tolerance to fire. Considered intolerant to very wet soils, but is reported growing in waterlogged soils (Peterken & Loyd 1967). Therefore the species is considered to have some tolerance to waterlogging.	MH	H
Reproduction
Reproductive system	Holly is a dioecious tree which reproduces sexually by flowering and producing fruit and vegetatively through suckering (Obeso 1998).	H	H
Number of propagules produced?	Each fruit normally contains between 4 and 5 seeds (Blood 2001). In Spain average fruit production per tree over a ten year period has been estimated in the range of 10,000 (Guitián & Bermejo 2006).	H	H
Propagule longevity?	After a three year experiment the initial seed bank was diminished by 80-90% and therefore holly is considered to have a short persistent seed bank (Arrieta & Suárez 2004).	L	H
Reproductive period?	Long lived (Blood 2001). An individual has been reported to be 254 years old (Peterken & Loyd 1967).	H	H
Time to reproductive maturity?	Under good conditions plants can reach maturity in 5 years (Blood 2001).	L	MH
Dispersal
Number of mechanisms?	Seed dispersed by birds and possibly by possums and foxes (Muyt 2001). Intact seeds have been reported to be excreted from captive possums in New Zealand (Williams et al 2000). Dispersed by bird species including blackbirds and to a minor extent by foxes (Garcia, Martinez & Obeso 2007). Seeds from Ilex sp. have been reported in regurgitated pellets of pied currawongs (Bass 1996).	H	H
How far do they disperse?	Reported to be dispersed by pied currawongs and by foxes which have been reported to have a dispersal range of 20 km and 5 km respectively (Bass 1996; Garcia, Martinez & Obeso 2007; Muyt 2001; Spennemann & Allen 2000).	H	H

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