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.

Vinca Replacement

Ever since we moved here 7 years ago I’ve wanted to replace the Vinca (V. minor) from our shady driveway turn-around.  Because I was new to the Mid-Atlantic area I didn’t want to do a wholesale replacement with something I wasn’t sure would work in this mostly shady, dry environment.  So I took a piecemeal approach, trying a little of this and a little of that, leaving most of the vinca in place.  This year I’ve decided to get more aggressive with the replacement, adding some successful species and trying some more new ones.

This was the next area for vinca removal.  It gets morning sun and open shade later in the day. 
It is framed on the right and left with test plantings of Heuchera and foam flower.

My primary reason for getting rid of the vinca is that it is an invasive species.  It is able to creep out of managed landscapes and run rampant in forested areas forming a dense ground cover that excludes native species.  From an aesthetic standpoint, while verdant, it can be rather boring and lack personality. Because of its ability to form thick foliage mats it tends to block out other less competitive species and creating a monoculture, at least on the ground plane.  

Most of the plants I have tried have survived the dry shade, but only a few have competed strongly against the viney invasive.  In all, I have tried nearly 25 native species in this area.  Strong competitors include Jack-in-the-pulpit (Arisaema triphyllum), green and gold (Chrysogonum virginianum), hayscented fern (Dennestadtia puntiloba), bigleaf aster (Eurybia macrophylla), Heucheras (I’ve had success with H. villosa and the cultivars ‘Citonelle’ and ‘Palace Purple’), twinleaf (Jeffersonia diphylla), Virginia creeper (Parthenocissus quinquifolia), zig-zag goldenrod (Solidago flexicaulis), merrybells (Uvularia sessilifolia), golden Alexanders (Zizia aurea), and the low-growing shrub sweetfern (Comptonia peregrina).  Also the non-native variegated Solomon’s seal (Polygonatum odoratum variagatum) is doing well (I imagine the larger native P. commutatum would also do well).  These are able to grow and spread without much help from me.  

Species that survive the conditions but need some help to keep from being overrun include wild bleeding hearts (Dicentra eximia), Alleghany spurge (Pachysandra procumbens), hoary mountain mint (Pycnanthemum incanum), smooth aster (Symphiotricum laevae), foam flower (Tiarella cordifolia), sedges (white-tinged sedge Carex albicans and pensylvanica, and there are some more robust wild ones like loose flowered sedge, C. laxiflora, that do very well).  Barren strawberry (Geum fragarioides, formerly in the genus Waldsteinia) should work well under these conditions, but in my case it seems to struggle.

Species that would be overrun without intervention include Meehan’s mint (Mehania cordata), woodland phlox (Phlox divaricata), and Labrador violet (Viola labradorica).  The only total failure was partridgeberry (Mitchella repens).  This tough little plant does not like to be covered, so between the vinca and the pine needles piling up it didn’t stand much of a chance. 

In this year’s planting I’m adding another sedge, ivory sedge (Carex eburnea), also wild geranium (Geranium maculatum) and, though I’m doubtful about this one, wild columbine (Aquilegia canadense).  I also have some downy wood mint, Bleiphila ciliata, which I started from seed that should do well in dry shade.

On the west end of this planting which gets a little afternoon sun, you can see the
ground hugging green and gold (now in bloom) and the much taller golden Alexanders (to the right).
Other plants include wild black raspberry, which appeared on its own, and several
volunteer trees that need to be pulled out.  

One of the features of vinca that is hard to copy with many native species is that it looks good (that is, pretty much the same) throughout the year with relatively little maintenance.  This is a great attribute from a design standpoint, but that does not make up for its invasive tendencies and relatively low ecological value.  Of the native species mentioned here green and gold and the sedges are evergreen (or nearly so) and the heucheras and foam flowers look good for most of the year.  Where there is sufficient moisture golden ragwort (Packera aurea) would be a very good evergreen choice.  

In the foreground you can see how Pennsylvania sedge pokes through gaps
 in the Virginia creeper (5-leaflets).  The sedge remains green throughout the year,
but looks a little ratty come January.  Another strong presence here is the zig-zag goldenrod,
with its ovate leaves, in the left and center of this image. 

Clearing this area took about 45 minutes.  Much of the time was consumed 
separating the vinca from the good soil after it had been pulled.  The pulled vinca 
was segregated to dry out and die before being put onto the debris pile.

Clearing the new planting space was surprisingly easy.  Since the area was thick layers of decomposed pine needles, the soil was very loose and most of the vinca could be removed with a 4-tine garden cultivator.  What didn’t come out with the cultivator was hand pulled.  I’m sure some bits of vinca remain and these will be addressed as they pop back up through the mulch.  I like using the fork because it hooked on the vines and it minimized damage to the deeper tree roots. 

Most of the new plants are in.  The most obvious are the heuchera and foam flowers. 
In addition to the wild geranium, columbine and ivory sedge are more
zig-zag goldenrod at the back of the planting and Labrador violets to the front edge. 
You can see the garden cultivator I used lying to the left.

Another means of vinca removal is cutting it back low then covering with a layer of cardboard, then mulch or clean soil.  This method would be better where the vinca is more firmly rooted.  I’ve tried spraying it with glyphosate, but the kill rate seemed rater low.

Here's the completed planting from another angle.  The new plants are mostly on the right edge
(see the little white tags?)  In the center of this view are established heuchera and foam flowers.

Using a variety of plant species in this area allowed me to tailor the planting to fit the variations in the site conditions: deeper vs. partial shade, arid vs. average moisture, etc.  Using plants with a tendency to spread both above ground, like foam flower and green and gold, or below the surface, like hay-scented fern helps cover the ground more quickly and allows the plants to migrate through the site to find their best conditions.  The variety also increases the biodiversity, extends the periods of bloom and provides more variation in form and color. 

I think for the next phase of vinca removal I will move to the middle of bed and try some taller species like the great Solomon's seal, white wood aster and more hay-scented fern.