Lonicera morrowii
(Morrow’s honeysuckle)

Lonicera morrowii is a deciduous, woody shrub, native to Japan, China and the Republic of Korea. It was introduced to the USA from Japan in the 1860s as an ornamental, but has since escaped cultivation, is considered invasive and is prohibited in some states in the USA. It invades open woodlands, old fields and other disturbed sites, and spreads rapidly due to seed dispersal by birds and mammals. It can form a dense understory thicket which restricts native plant growth and tree seedling establishment. L. morrowii hybridizes with another non-native honeysuckle, L. tatarica, to produce L. x bella, and this plant is also considered invasive.

L. morrowii is native to Japan, China and the Republic of Korea (USDA-ARS, 2018). It is introduced in Canada, occurring in British Colombia, New Brunswick, Ontario, Quebec and Saskatchewan (Canadensys, 2017). It is also introduced in the USA, where it is listed in many states as an invasive species (Go Botany, 2018; USDA-NRCS, 2018).

Where this species is considered to have ornamental value, there is the possibility of it escaping cultivation and spreading in the wild.

In the USA, L. morrowii is found in disturbed habitats, floodplains of rivers or streams, in forests and on the edges of wetlands (Go Botany, 2018). It is commonly found in forests and edges, as well as old fields and roadsides (MA-DACF, 2018). 

In New England, it is found in abandoned fields, early successional forest, forest edges, floodplain forest, open disturbed areas, pasture, planted forest, roadsides, utility right-of-way, vacant lots, yards and gardens (IPANE, 2018).

Genetics

L. morrowii has a chromosome number of 2n = 18 (Prof. Summer’s Web Garden, 2009).

It has been reported to hybridize with L. tatarica to produce L. x bella, a hybrid frequently observed in Connecticut, Massachusetts, Maine, New Hampshire, Rhode Island and Vermont (Go Botany, 2018).

Reproductive Biology

L. morrowii produces an abundance of seeds that are long-lived in the soil (TexasInvasives.org, 2004). It has been suggested that the seeds require a cold stratification period to break dormancy (Hoffman and Kearns, 1997). However, some studies suggest that seeds may require a warm stratification period or no stratification period at all to germinate (Hidayati et al., 2000). Fresh seeds of L. morrowii are not always dormant, with about 30% germination at 25/15˚C in light (Hidayati et al., 2002).

Physiology and Phenology

L. morrowii flowers between May and June (Hoffman and Kearns, 1997; IPANE, 2018).

In the USA, L. morrowii hybridizes with L. tatarica, to produce L. x bella (Whitehead and Bowers, 2013; Go Botany, 2018). It has been shown that the hybrid inherits a diversity of glycosides (components of plant defence compounds) from its parents, but also has some that are not present in either parent (Whitehead and Bowers, 2013). These compounds have consequences for plant interactions with pathogens, herbivores and seed dispersers and so could affect potential invasiveness (Whitehead and Bowers, 2013).

L. morrowii and the hybrid L. x bella have smaller genome sizes and higher photosynthetic capacity than the native species L. canadensis, traits that have been linked to growth behaviour and invasiveness in woody species (Fridley and Craddock, 2015).

Associations

A study by Edalgo and Anderson (2009), carried out in an old-field area in Pennsylvania, USA, found that there was a higher abundance of exotic earthworms under L. morrowii than under the native species Viburnum dentatum. The authors suggested that native shrubs could be useful for revegetation plans to reduce the abundance of both exotic shrubs and earthworms.

Environmental Requirements

In its native range, L. morrowii occurs in climates similar to those on the Atlantic coast of the USA (UMaine Extension, 2001). It prefers mesic soils, but grows well in dry, sandy soils in calcareous areas (IPANE, 2018). It tolerates nutrient-poor soils (MA-DACF, 2018). It also prefers sun, but is shade-tolerant and can invade intact forest understory (MA-DACF, 2018).

L. morrowii and its hybrids are particularly susceptible to attack by Hyadaphis tataricae (Herman and Davidson, 1997). H. tataricae (European honeysuckle aphid) is an introduced pest to the USA and controls flower and fruit production in some bush honeysuckles (Nyboer, 2007). Heavy infestations may cause branch tips to be deformed and develop witches’ brooms, which may reduce fruit production (Nyboer, 2007). Native ladybird beetles have been noted to control this aphid (Nyboer, 2007).

Environmental

• Erosion control or dune stabilization
• Wildlife habitat

Ornamental

• garden plant

L. morrowii is a bush honeysuckle, and so is a stout, erect shrub, whereas species native to the USA, including grape honeysuckle (L. reticulata), yellow honeysuckle (L. flava), and red honeysuckle (L. dioica), are woody vine-like twining species (Hoffman and Kearns, 1997).

L. morrowii is similar to L. canadensis (fly-honeysuckle), also native to the USA, although this has a solid pith, hairless leaves, tubular flowers without long splayed petals and oblong reddish fruits (MA-DACF, 2018). L. morrowii is also similar to Diervilla lonicera, L. maackii and L. tatarica. D. lonicera has a solid pith compared to the hollow pith of L. morrowii, and its leaves have fine teeth and are long-pointed at the tip (MA-DACF, 2018). L. maackii has peduncles shorter than 5 mm, compared to 5-15 mm in L. morrowii. It also has leaf blades that taper abruptly to a narrow point, whereas in L. morrowii the leaf blades are less pointed at the apex (Go Botany, 2018). L. tatarica can be distinguished from L. morrowii, as the former does not have hairs on the under surface of leaves and has an inflorescence stalk of 15-25 mm (compared with 5-15 mm in L. morrowii) (Go Botany, 2018). In contrast to L. morrowii, L. tatarica has pink to white flowers that do not turn yellow with age (Snyder and Kaufman, 2004). L. morrowii hybridizes with L. tatarica to produce L. x bella Zabel. This hybrid is morphologically variable but usually displays a pink (fading yellow) corolla that is scarcely saccate at the base, sparsely pubescent branchlets and leaf blades, and peduncles 5-15 mm long.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

SPS Measures

L. morrowii is categorized as a noxious weed in 46 states of the USA (USDA-NRCS, 2018).

According to the USDA Plant Database (USDA-NRCS, 2018), L. morrowii is classed as invasive and is banned in Connecticut. It is prohibited in Massachusetts, is a prohibited invasive species in New Hampshire, and is categorized as a Class B noxious weed in Vermont, USA (USDA-NRCS, 2018). Go Botany (2018) report it as invasive and prohibited in Connecticut, Maine, Massachusetts (where the hybrid L. x bella is also prohibited), New Hampshire and Vermont, and it is classed as invasive in Rhode Island.

Along with its hybrid, L. x bella, it is ranked as a severe threat in Pennsylvania (PA-DCNR, 2018). In Kentucky, L. morrowii is also ranked as a severe threat (KY-EPPC, 2013). Both L. morrowii and hybrid L. x bella are on the alert list for Tennessee (TE-EPPC, 2009), where they are considered pests and are not permitted to be propagated, sold or released within the state of Tennessee (Tennessee Department of Agriculture, 2018). Both L. morrowii and L. x bella are restricted in Wisconsin (WDNR, 2018), which means they cannot be transported, transferred, or introduced. Possession is allowed and control is encouraged, but not required. The Midwest Invasive Plant Network also recognize this species as invasive in Missouri and the Midwest and recommend that it should not be planted (Missouri Botanical Garden, 2018).

Control

Control of L. morrowii may require one or more of the following: prescribed burning, hand pulling of seedlings, cutting and herbicide treatments (Nyboer, 2007).

Physical/Mechanical Control

Small L. morrowii plants and seedlings can be pulled up by the roots when the soil is moist (MA-DACF, 2018). The entire root needs to be removed, or resprouting will occur (Nyboer, 2007). Larger plants can be cut but, again, resprouting will occur unless the root is removed (MA-DACF, 2018). Physical hand removal of small plants and grubbing out of large plants are methods that should not be used in sensitive habitats (Hoffman and Kearns, 1997; Nyboer, 2007). Open soil and remaining root stocks will result in rapid reinvasion or resprouting of honeysuckles and other exotics (Nyboer, 2007). Soil should be tamped down to discourage further establishment of seedlings (Hoffman and Kearns, 1997). After clearing L. morrowii from an area, re-invasion may be reduced by planting the area with native species (Hoffman and Kearns, 1997; Love and Anderson, 2009). Mowing may prevent seedlings from establishing, but repeated heavy damage over several years is required to kill established shrubs (MA-DACF, 2018). In fire-adapted communities, prescribed burning in spring kills seedlings and the tops of mature L. morrowii plants, although this may need to be repeated annually for up to five years due to resprouting (Nyboer, 2007).

In trials comparing four control methods for L. morrowii in Pennsylvania, USA, it was found that foliar applications of glyphosphate and mechanical removal were more effective at reducing shrub numbers than cutting and stump applications of glyphosphate (Love and Anderson, 2009). Spring treatments, particularly cut and mechanical treatments, produced plots with higher herbaceous community quality (Love and Anderson, 2009). It is suggested that follow-up treatments, control of white-tailed deer (O. virginianus), and planting of native seeds and saplings should also be used to increase the establishment of native plants (Love and Anderson, 2009).

Biological Control

Goats and sheep browse L. morrowii (MA-DACF, 2018).

Chemical Control

Herbicides are effective as foliar applications (2% glyphosate or 3-5% triclopyr solution), cut-stump application (20-25% glyphosate or triclopyr solution applied immediately after cutting), or basal bark application (20-25% glyphosate or triclopyr ester) (MA-DACF, 2018).

L. morrowii stems can be cut at the base with brushcutters, chainsaws or hand-tools, and a solution of 20% glyphosphate can be applied to the cut stump with a sprayer or by wiping with a sponge applicator (Nyboer, 2007). Two cuts per year (one in early spring and one in early autumn are recommended, and if they are not followed with herbicide treatment, vigorous sprouting will occur when the plant comes out of dormancy (Hoffman and Kearns, 1997). Application in the late summer, autumn or during the dormant season appears to be effective (Nyboer, 2007).

Triclopyr formulated for water dilution is not effective, although when formulated for dilution in diesel fuel it can be used on cut stumps throughout the year, with winter application proving to be more effective (Hoffman and Kearns, 1997). In trials in Connecticut, USA, triclopyr treatment reduced the number and length of sprouts in cut stumps of L. morrowii (Mervosh and Gumbart, 2015). In the same trials, glyphosphate completely prevented sprouting.

Ecosystem Restoration

Modeling techniques are being developed that will allow land managers to prioritize invasive shrubs (including L. morrowii, L. tatarica and L. x bella) for removal, and that can determine replanting sizes for native plants (Hassett and McGee, 2017).

There is a lack of information about specific threats to endangered native species. Information about the use of specific biological control agents is also lacking.

03/06/18 Original text by:

Vicki Cottrell, Consultant, UK