Dealing with Invasives in the Winter

Fall and winter are good times to use herbicides to control invasives.  At this time it is easier to spot many invasive species because they often hold onto their leaves longer than native species and there is less likelihood of harming native species with any mis-directed herbicide.  However not all invasive species can be treated effectively during this period.  At their January meeting, Blue Ridge Prism, an organization based in Virginia to address the threat of invasive species and to restore native habitats, the topic was about which plants could be treated over the winter months. A recording of that meeting can be found at this link:  January Meeting.  For each species mentioned here there are links to Blue Ridge Prism's  Fact sheets .  These provide lots of information each species including how to identify them and others species that may look similar.

One of the most obvious targets for winter actions are invasive vines growing up trees.  These include  English ivy, Hedera helix, and winter creeper, Euonymus fortunii.  At a minimum, these vines should be cut at the base of the tree and again a few inches to a foot or so up to create a gap.  If possible the lower stump of the vine should be treated with a 50% dilution of a recommended herbicide (e.g., 20% solution of glyphosate) to kill the roots.  This is referred to as the cut stump treatment.  The remaining vines in the tree should be left in place to slowly die.  Attempting removal can damage the tree and/or injure folks on the ground with falling debris.  Be observant when cutting the vines, wintercreeper vines especially are good at hiding in the furrows of the bark of trees. 

This tree has surrendered tp English Ivy.  The added weight
 can cause limbs to break, particularly with
accumulated snow and ice in winter.

These trees have winter creeper euonymus climbing up.  Winter creeper
sends out longer branches than English ivy.  These branches will bear
flowers resulting in the spread of this invasive species. 
Note that both of these vines have been 'deer pruned',
 but doesn't keep them from climbing.

For plants growing along the ground, these evergreen vines can also be treated with foliar sprays in winter, though not with 100% success.  Digging when the ground is soft and moist is quite effective at any time of year.

While not expressly discussed, Japanese honeysuckle, Lonicera japonica, vines cane also be treated in late fall and winter by either cut stump or foliar spray, if leaves are present.

Winter foliar spraying of  Vinca (V. minor and major) was reported to be effective, though personally I have not seen satisfying results.  For small infestations hand pulling is very effective.  I find that using a rake to lift most of the stems followed by using a trowel to get out the rooted plants is pretty effective.  Many folks recommend cutting, mowing or weed whacking first, then, later, hitting the regrowth with a foliar herbicide.  In doing this one must be very careful about killing non-target plants.  (These herbicide treatments are likely to be during a time while desirable plants are actively growing as well.)

There is not much else growing around this patch of Vinca minor
(Periwinkle) in January so careful foliar spraying shouldn't damage other plants.

I do have several areas where vinca has escaped the beds and is moving into forested areas.  Later in February I will try an experiment with foliar spraying alone compared with string trimming + raking followed by a spray with glyphosate.  There are some natives in this area, such as white avens, so I don't want to wait too long before I get started on this.

Fall and winter are good times to treat garlic mustard, Allairia petiolata.  The leafy rosettes of overwintering plants are easy to spot treat with the appropriate herbicide when other plants are still dormant.  Treatments can be done when temperatures are above 40°F.  I've done treatments in November and February-March. In my opinion the treatments later in winter were more effective because the leaf litter had broken down more making the garlic mustard leaves easier to target.  See this link to an earlier blog post about my efforts to eliminate garlic mustard.

Here's some garlic mustard in November,  In a mild winter
 these leaves will remain active all season.  Around March
 they will begin a growth spurt and put up a flowering stalk.

Gill-over-the-ground or creeping Charlie, Glechoma hederacea, can be treated with foliar sprays in early winter before a hard frost, but these would be more effective in early fall while plants are moving nutrients to their roots.  By January  most of the leaves have fallen off so there is no good target for a foliar spray.  Multiple treatments will likely be necessary.  The advantage of winter spraying, again, is that there are few non-target species actively growing at that time.

For Autumn olive, Elaeagnus umbellata, Nandina, Nandina domestica, and Bradford pear, Pyrus calleryana (fall to early winter), cut stump, hack and spray or basal bark treatments are effective. Autumn olive and Nandina can be treated effectively through the fall and winter. Treatment is not so effective in early spring when sap is flowing upwards.  Bradford pear should be treated earlier, while sap is still flowing downwards to the roots.  Smaller plants can be cut close to the ground and immediately treated with a concentrated herbicide on the stump.  Larger specimens can be treated using basal bark treatments (herbicide in an oil carrier) or hack and squirt.

These autumn olive bushes are in full bloom with their sweetly scented
 flowers seen here in early May.  I eliminated both of these with the
cut stump method using glyphosate.

While some invasive species can be treated throughout the winter months, usually when temperatures are above 40°F, some require treatment in a more specific time period.  Mid-fall to early winter is a time in which sap is flowing from the upper parts down to the roots.  Herbicide treatments at this time will carry the active ingredients to where they will have the greatest effect.  As mentioned above Bradford pear and gill-over-the-ground should be treated earlier.  Tree-of-heaven and Asiatic bittersweet are also in this group.  

Tree-of-heaven, Ailanthus altissima, is best treated in early fall when the sap is following down to the roots using basal bark or hack and squirt methods.  Cutting the this tree down is not an effective means of control.  The response to being cut down is for it to put out many root sprouts, so even treating the stump with herbicide is not going to be effective. 

Italian arum, Arum italicum, does not have an effective approved herbicide so digging is currently the only effective means of removal.

Lesser Celandine, Ficaria verna, is a major problem in moist areas, particularly along streams.  These can look like violets when not in bloom.  Digging small infestations can be done all year but care must be taken to remove all the roots, tubers, and bulbils, as well as seeds, if present, to avoid recurrence or spreading of this species.  There is a very narrow window of opportunity (about 2 weeks) for effective foliar treatment with glyphosate.  This is in the March-April time frame shortly after the plants have started to bloom, but before full bloom is achieved.  In wetland areas so you need to use a herbicide approved for those places (e.g., Rodeo®). 

This photo of lesser celandine was taken in mid-April toward
the end of the herbicide application window. 
When not in bloom the leaves do look similar to many violets

Some General Comments

Make sure you can positively identify the target species.  Especially with shrubs.  You could always tag the plant of interest with florescent tape in the winter and come back to it when it's leafed out or in flower to do a positive ID.

If you don't wish to use herbicide treatment, winter is a good time for pull up plants at times when the ground is thawed and moist and plants can be easily removed.  Just be careful not to disperse any seeds associated with the plant that you are removing.

For most plants a form of glyphosate will work.  Check the links for specific guidelines for each species.  In general, I prefer glyphosate as it is not translocated in the soil to other plants and that which reaches the soil is broken down quickly by soil bacteria.  In all cases follow the herbicide label instructions.

Just to throw in my own two cents, I wanted to add two things.  First while there is nothing you can do with Japanese stiltgrass  in winter, early spring (when forsythisas are blooming) is when you can put down preemergent herbicides to help eliminate this from lawns and areas where you are not encouraging growth from seeds.  Check this post for details.

Second, winter is also a good time to treat Japanese barberry using a cut stump treatment.  It can be spotted by the few remaining red berries along the stems.  There is a native barberry but it's quite uncommon.  It differs from Japanese barberry in that it has spines in groups of three along it stem, rather than the singular spines on Japanese barberry.

Dealing with Invasives in Winter

This is a pretty typical scene when English ivy gets established
in a tree.  This ivy is robbing light from the tree and also weighing
 it down, leading to limb breakage.  Cutting a section of each vine
around the base of this tree will kill all the vine above the cut in a month or so.

Wintercreeper grows up trees with the aid of sticky rootlets. 
Its evergreen foliage shades the host trees as well as
the surrounding area.  In areas with plentiful deer the bottom 4'
 are often stripped of foliage.  The same can be seen with English ivy.     

Winter is a very good time to have an impact on Mid-Atlantic invasive plants.  Many of these still have leaves and are susceptible to herbicide treatments.  It’s also easier to see where plants are (though identification can be trickier). Most native species are dormant during winter so there is less risk of damage from herbicides or from tromping through the landscape.  In can be more comfortable to work in cooler weather.  As long as it is above 45°F, many herbicides are still effective (see below). 

When I look out at unmanaged areas infested with invasive plants I see many opportunities to have a large impact on reducing the spread of many of these species without a lot of effort.  For some species just keeping them from climbing up the tree will have a huge impact on controlling their spread.  OF course complete removal for most of these species will take several years of consistent effort, but just keeping them from climbing trees can limit their spread and be much healthier for your established trees and large shrubs. 

Among the species that need to climb in order to bloom and produce seeds are English ivy, Japanese honeysuckle, winter creeper, and oriental bittersweet.  These vining species produce flowers when they are growing upwards or over the tops of other plants.  You can significantly reduce their seed production by keeping them from growing upwards.  Most simply this can be done by cutting the vines growing into trees and larger shrubs close to ground level.  Applying the appropriate herbicide to the stump will go one step further in eliminating that plant from the landscape.  If you aren’t using herbicide, clear the invasive from a zone around the base of a tree to help slow re-establishment of the invasive vine. 

The bittersweet, wisteria and honeysuckle grow upwards by twining around their hosts.  These vines grow tightly around the trunks and branches of their host plants in effect strangling them.  While English ivy and wintercreeper don’t twine as much (they climb with the help of sticky rootlets growing from the stem) its dense evergreen foliage gets very heavy, especially in winter, and can bring down branches or even whole trees.

Oriental bittersweet can grow vertically by twining around itself. 
Once it finds a suitable host it will continue upward
growing around the host.  I cut this one high last spring so
that I could come back later and treat a fresh cut with herbicide.

This Japanese honeysuckle was cut last season.  You can see
 the damage caused by this tightly twining vine.

The table below summarizes how to treat several of the invasive species in fall and winter.  All this information was taken from the references cited.  For details check out the links to the references for each species.  Before using herbicide read and follow the label directions.  Don’t forget to wear proper protective equipment.  Another important safety practice in dealing with vines is to NOT pull them out of the trees after you cut them.  You risk damaging the tree and yourself.  They will dry up and fall out on their own.

Invasive	Common name	Winter treatments	timing	Reference
Celastrus orbiculatus	Oriental Bittersweet	Cut stump treatment with 20% glyphosate or  trichlopyr. Digging/pulling partially effective, but plants can regrow from root fragments.	Fall-Winter when temps above 40°F.   Digging can be done anytime.	Bugwood CO
Lonicera japonica	Japanese honeysuckle	Foliar treatments with glyphosate (0.75-1.5%) effective.  Later in season higher concentrations are more effective.  Use for easily accessible foliage (ground level).	After first frost but before hard frost are very effective.  Mid-winter treatments were less effective.	Bugwood LJ
		Cut stump (25% glyphosate) for climbing honeysuckle vines	Cut stump most effective June-Winter	PRISM
Euonymus fortunei	Wintercreeper euonymus	Cut back climbing vines to prevent flowering and fruiting.  Small infestations can be dug out, but plants can regenerate from stem fragments.	Anytime, the sooner the better.   Flowers form in summer with ripe fruits in fall on climbing vines.  Dig plants when soil is soft and easily worked.	Forest Service
		Cut stem treatment with 25% glyphosate for climbing vines. Foliar spray with glyphosate or triclopyr (2%) for large infestations on ground	When temperatures are above 40°F.  Treat immediately after cutting. Mid to late fall when other species are dormant.  Best at or above 65°F.	TN Exotic Plant Management Manual
Hedera helix	English ivy	Cut vines growing up trees close to ground and again 1-2’ up.  If possible, treat cut stump with 25% active ingredient glyphosate or triclopyr amine.	Cut stump method is effective year round.  Herbicide treatment best when temperatures are above 55°F.	Invasive.org
		Foliar treatment with these chemicals (2-5%) when temperatures are above 55°F are partially effective.	Foliar treatments most effective from mid-summer through fall.  Partially effective in winter.  Apply during mild periods (above 55°F) while other plants are dormant.
		Smaller non-climbing infestations can be manually removed.  Plants can resprout from any remaining roots or vine.	Pulling can be done anytime.  There are fewer competitive plants in winter.
Wisteria sp.	Wisteria	Cut vines about 2” from the ground.  Treat with 25% glyphosate or trichlopyr.  Plants will resprout if not treated with herbicide. Small infestations can be dug out, but resprouting is possible from pieces left behind.	Cut stump treatment can be done anytime that the ground is not frozen.	Bugwood

NOTE: In most jurisdictions a home owner can apply OTC herbicides to their own properties, but they are restricted from doing so on public lands or on another person’s property.  In most cases a certified pesticide applicator is need to apply herbicides to any property but your own.

Before attacking the invasive species it is important to be able to know which plants are desirable and which are not.  Some species are easy to identify in winter (e.g., English ivy and wintercreeper).  Others like oriental bittersweet, can be difficult to distinguish from their domestic relatives.  Check out this guide from the Delaware Department of Agriculture for help in identifying invasive plant species.  Of course any vine that is threatening the life of a desirable tree or shrub is a candidate for removal.  

This adolescent hickory tree sustained damage from
Japanese honeysuckle a few years back.  The truck is swollen
due to sap being restricted from flowing up the tree.  I'm not sure
if this tree will continue to survive into maturity.

In looking over the nearby woodlands I noticed that most of the native vining species do not spiral tightly around their host.   Deciduous native vines like Virginia creeper and native grapes do not twine so they do not strangle trees, and, since they lose their leaves in the fall they do not add a lot of weight through the winter.  Native vines growing high into trees can reduce the amount of light penetrating the tree canopy.  One of the bigger problems on my property with vines growing up in the canopy is that they join trees together so that when one falls it can damage other trees that are connected through the vines.  An important benefit of these native species is that their berries are an import food source for many birds, poison ivy included.

Catbrier (Smilax sp.), the green stems here, is a native species that twines
loosely on itself as well as on other nearby plants.  While it can
 be a nuisance due to its stiff thorns it plays well with other established plants.

Another group of invasive species that can be attacked in winter are woody shrubs and herbaceous species that are still green and active in the winter months.  These include multiflora rose (Rosa multiflora), wineberry (Rubus phoenicolasius) and garlic mustard (Alliaria petiolata).  Last year I wrote a more detailed post  on how to deal with these species in the winter. 

As an update to that post, I did test whether burning the crowns of wineberry with a garden torch was sufficient to kill it.  In most cases the answer was no.  Most of the crown resprouted later in the spring. Another action took was to  cut back the long arching branches in late summer and fall.  This shoud reduce its spread since it can form roots wherever a branch tip contacts the ground.  The effectiveness of this effort will be difficult for me to measure but I hope to get a sense if there are fewer of these out there.

For a general overview of Mid-Atlantic invasive species see Plant Invaders of  Mid-Atlantic Natural Areas  For detailed instructions on how to treat you should look to nearby state resources or University cooperative extension services.

What else can you do?

Aside from managing your own property there is not a lot an individual can due by themselves or without permission.   There are a number of ways to volunteer to do plant conservation or get involved with removal of invasive species.  I first got started working with native plants by becoming a plant conservation volunteer with what was then the New England Native Plant Society (now the Native Plant Trust).  A number of our projects involved clearing out invasive species from public and some private lands.  We were trained on how to identify our target and how to remove it.

To work on public lands you need training and or supervision.  Contact the public lands supervisor for the areas where you want to help.  Some other places where you can look for opportunities are plant conservation groups and state or regional native plant societies.  In Maryland there are several counties with ‘Weed Warrior’ groups (for example see  Weed Warriors  for Montgomery County).  In Northern Virginia there are the Tree Rescuers.  Also in Virginia there is  Blue Ridge Prism, a group dedicated to removing invasives from the forests of the Blue Ridge Mountains.  The USDA website has some general information including a few specific links to projects around the country.  The Nature Conservancy has a volunteer site that can be searched by location and date.