Dogwood Dilemma

by Don Osmond

Dogwood is one of the most difficult woody invaders to control in prairies because it has a deep bag of tricks.  Like many woody plants, it readily resprouts when grazed, cut or burned, sometimes producing more stems after the disturbance than before.  It likely has access to deeper water sources than prairie grass, allowing it to ride out droughts.  The biggest advantage it has is combining a large clonal root system that resists herbicides with a substantial underground reserve of starch, the fuel needed to resprout after disturbance.

I recently tried to control dogwood in a remnant & failed miserably.  This post is my attempt to tap the scientific literature to hopefully inform a new approach that reduces the effort needed over time rather than increasing it, as well as suggesting needed experiments.  I used Google Scholar to locate 92 open access studies & 38 abstracts (full text behind a paywall).  This included a small number of Cornus racemosa & drummondii studies, papers on other deciduous clonal shrubs to get an idea of general woody plant behavior, studies of woody plant resprouting mechanisms, studies on root storage dynamics & discussions on woody plant encroachment in grasslands.  I weighted the studies using various criteria & combined them with a small number of high quality field reports.

My treatment failure

In 2022-23 I performed dormant season cut stump herbicide treatment on multiple patches that were somewhat isolated from each other, but it’s hard to know what is going on underground, so I may not have treated the entire clone, as there are other patches in the area.  20% Garlon 4 equivalent in bark oil was meticulously applied with a low pressure stream nozzle to all but the tiniest stumps with no snow on the ground & 48 hours precipitation free after application.

Dogwood (seen in the foreground) is scattered over much of the remnant.

One of the shrub walls where I cut and stump-herbicided (dogwood has red leaves).

The next growing season all was good.  Very few resprouts occurred, mostly at patch edges where there were some scattered uncut plants outside the patch.  Perhaps the root system outside the patch counteracted herbicide on the patch fringes.  Even though I looked carefully, it’s possible I missed tiny resprouts in the thatch, so this summer I’ll look harder at a clone I cut stump herbicided in fall 2024.  The positive result of few resprouts was a go signal to continue cut stump the next dormant season of 2023-24.  The site was burned in March 2024.  What I saw in August was demoralizing.  There were resprouts, which didn’t surprise me because fire triggers resprouting in woody plants.  But it was widespread & abundant in patches treated 0.5 years ago as well as 1.5 years ago, much different than a year before.  It’s as if the burn had counteracted the herbicide effects (by a nutrient pulse?).  Of course, I don’t know what the result would be if no burn had occurred.  Perhaps the herbicide “wears off”?  Or the combination of herbicide & the shading of dense grass prevented resprouts the year before?  Did the burn stimulate rhizome buds that were not stimulated by cut stump herbiciding?  Lots of theories.  One thing I noticed is resprout density was lower, sometimes zero, in the densely populated patch center.  My theory for that is the denser, larger diameter stems in the patch center translocated more herbicide to the root than the more scattered, small diameter stems in the patch fringes.

 

What is NSC (nonstructural carbohydrates)? (also called TNC: total nonstructural carbohydrates)

Control of dogwood was not going to be easy, so I needed to get acquainted with the plant I was dealing with & how it functions.  For most plants, leaf photosynthesis leads to the production of NSC, mainly consisting of sugars & starch, which is used for many plant processes including storage.  The main purpose of storage in the form of starch is to provide fuel for growth when leaves are not present due to dormancy (e.g. for bud break & initial leaf out) & in the event disturbance removes or hinders photosynthetic ability.  Examples of disturbance are grazing, burning, cutting, disease, insect defoliation and drought.  In clonal plants, injury to only a portion of the clone could remobilize NSC from the nearest uninjured stems (which also store NSC for short term use) rather than the main rhizome.  Therefore such injury may not affect NSC stores, but I couldn’t find a study that confirms this.

 

Seasonal variation of NSC in root storage pools

Many species of woody trees & shrubs have a similar pattern.  In early spring, root NSC concentrations drop to a minimum to supply the resources for leaf emergence & expansion.  Then there is a rapid increase as the resprouts or branch leaves start photosynthesizing, sending resources to the roots, replenishing the low NSC reserves.  The increase continues until it reaches a peak in mid-summer to early fall (depending on the species), then slowly decreases over the dormant season as it supplies respiration needs.  We are interested in the minimum seasonal level because theoretically, removing stem photosynthesis at that time means root NSC will be tapped to fuel new resprouts, lowering root NSC even further.  The hope is that yearly cutting at that time will reduce NSC in steps until the plant dies or is severely weakened.  NSC minimum in Cornus racemosa occurs when the flowers start opening around June 1, but since NSC rapidly rises after that point & growth stages vary year to year, scout often to nail the date for cutting.

 

Can we use sumac control techniques on dogwood?

Since both of these often co-exist on a site & both are clonal, it’s tempting to think they respond to disturbance similarly.  But there is a major difference between them.  Dogwood is on the phalanx end of the phalanx-guerilla spectrum for clonal woody plants while sumac is on the guerilla end.  The guerilla strategy is to quickly grow rhizomes relatively far from the central clone to test the suitability of habitat.  The outer stems are spaced relatively far apart & tend not to resprout when cut.  The inner stems do resprout & produce seeds.  Phalanx is a slow advancement of closely spaced stems.  The inner stems produce seed while outer ones conquer territory.  Also, a study found the 2 had very different above ground carbon allocation, drought tolerance & photosynthetic capacity.  So sumac & dogwood are likely to require different management techniques.  For reference, multiple people have had success eradicating sumac clones by cutting just after flowering when root NSC is lowest (around July 1), then cutting the resprouts around August 1 for several consecutive years.

 

What are the results of single or multiple cuts in the growing season & cutting for multiple consecutive years?

Cutting in the dormant season is not effective since dogwood roots contain more than enough NSC to resprout in the spring.  Cutting should be on the entire clone at once, to prevent photosynthesizing stems in untreated parts of the clone from sending resources to the treated part.  Clones can grade into each other, making it difficult to know where one ends & another begins.  Stems can store some NSC needed for resprouting,  so cut them close to the ground.  Leave as much ground vegetation as possible to help shade resprouts.

 

I have no anecdotal reports with enough detail & found only one useful study on cutting dogwood.  The study was only 2 years long & didn’t treat entire clones.  2 consecutive years of cutting somewhat near the date where root NSC was minimum, then allowing one year of rest, resulted in root NSC returning to pre-treatment levels.  That seems to indicate failure, but resprout growth was slow, which could indicate low NSC reserves before the rest period, or that the plant was actively allocating more resources to storage than to growth due to the disturbance.  The second year of cutting produced less % cover than the first year.  So there are small indications that cutting was hurting the plant.  If they treated the entire clone at once & more carefully nailed the seasonal timing, their results would show more success.  I think a single summer cut at the time of minimum root NSC for at least 3 consecutive years is worth trying.  Installing a few small study plots will help you assess results by annually taking a picture & measuring average stem height, diameter & density.  I wouldn’t adopt this strategy for large areas until experiments indicate it works in conjunction with burning.

 

Cutting twice in the growing season for multiple years should theoretically be better than cutting once, but I found no studies on it.  It works for sumac, but as I pointed out earlier, that plant likely requires different management techniques than dogwood.  The first cut should be timed for minimum root NSC.  It’s unclear when the second cut should be (see the section below on seasonal timing) and it’s unknown if there will be any resprouts in the months following the 1^st cut.  Since I’m too busy with other weeds in the growing season, I’m not considering cutting twice in a year.

 

More on the timing of summer cutting

Besides cutting when root NSC is minimum, another strategy that works in some species is to cut late in the growing season.  Studies admit it’s unknown why this sometimes works & they say more research is needed.  One theory is this method can interfere with the plant hardening off before freezing temperatures arrive, causing injury or death.  Because so much is unknown, I don’t think experimenting with this method is a good use of scarce resources.

 

A Kansas study on Cornus drummondii:  An early May burn shifted the time of NSC minimum from late May to about July 1.  It’s unknown if earlier spring burns would have the same effect.  So cutting after a spring burn should perhaps be delayed until the resprouts fully open their leaves.  In addition, root NSC was lower after a spring burn compared to unburned but a New York study with a late April burn didn’t show this difference.  So it’s unknown if cutting after a spring burn is more effective compared to cutting without a burn.

 

What about burning?

As with most shrubs, fire increases stem density.  A burn interval of over 2 years may be effective in preventing the establishment of some shrub species, but if in the past that interval increased temporarily, shrubs can establish & once that happens, you need intervals shorter than 2 years to prevent expansion.  Even annual burns will probably not reduce clone coverage in some cases, but will likely stop expansion.  Bill Kleiman at The Nature Conservancy Nachusa Grassland has kept dogwood density acceptable in one location by one summer mow followed by an average 1.5 year burn interval for 30 years.  Rich Henderson of The Prairie Enthusiasts reports 30 years of near annual burning of an 8’ diameter clone resulted in only a few tiny stems.  That indicates even small clones with relatively immature root systems are quite fire resistant.  Each site will need to choose burn intervals based on factors like bird/insect populations & resource availability.  Cutting dense clone centers before burning will gradually allow an adequate intensity of fire to reach all parts of the clone.

 

Herbiciding

The few studies I found on herbiciding dogwood used small treatment plots that included only part of the clone, so it’s likely untreated stems outside the plots provided resources to roots in the plot, making results worse than if the entire clone was treated.  They also didn’t monitor for a long enough time to prove roots were killed.  So I couldn’t use their results.

I also couldn’t use some anecdotal reports due to not mentioning whether entire clones were treated & lack of long term follow-up monitoring.  Here are some of the more useful experiences:

·      Rich Henderson of The Prairie Enthusiasts reports glyphosate cut stump is more effective than triclopyr, killing stems 6-12” away from the application point, indicating good translocation.

·      A contractor reported cut stump herbiciding followed by 3 consecutive years of burning reduced it to the point where normal burn intervals will keep it in check.  While many sites cannot do annual burning, this indicates that a single initial herbicide treatment could injure the clone enough that repeated annual disturbance (maybe substitute herbiciding or growing season cutting for fire in non-fire years) could lead to a permanent less intense invaded state.

·      Stephen Packard reported dormant season basal bark or cut stump worked well on small, low density populations, but there were resprouts in denser ones.  He said multiple years of herbicide will likely be needed.

·      The late Tom Vanderpoel of Citizens for Conservation was very experienced & was bullish on dribbling herbicide on the stems in the growing season without the liquid reaching the ground, using only the best people & only in remnants due to labor intensiveness.  For degraded areas he mowed & followed up with foliar herbicide.

·      An anecdotal report from The Nature Conservancy Indiana found excellent control of Cornus racemosa by dormant season cut stump herbiciding followed by spring burning after bud break, repeating as many years as necessary.  For most sites, burning at that time is undesirable or impossible, but the concept of weakening the plant with herbicide & then hitting it again with another stress when it starts growing is interesting.

 

Discussion & Conclusion

·      Foliar spray is worth trying in degraded areas with no desirable natives.  Theoretically the best time for foliar is when NSC is moving from photosynthesizing leaves to the root.  For dogwood, this is mid-June in New York.

·      Dormant season cut stump is very labor intensive for dogwood, but it removes biomass, which allows less moist conditions in dense clones, probably improving fire behavior.  With the proper techniques, proper equipment & attention to precipitation events, there will be no off target kill.  In one year, I saw dogwood sap flow with cut stump starting 4/11 but not on 4/26.  So there is a narrow window when cut stump should not be performed.  Based on Rich Henderson’s experience, more experiments are needed to replicate his results with glyphosate.

·      Basal bark is also worth trying & is also labor intensive, but not as much as cut stump.  However there are serious drawbacks using such a high herbicide concentration, especially if the applicator is not methodical & careful.  If precipitation occurs within 4 days of application, enough herbicide can wash into the soil to cause a dead zone, even if the application is in the dormant season.  Weather forecasts 4 days out are wrong often enough.  Basal bark application in remnants must be done with a wick, squeeze bottle or paint roller instead of a sprayer, but it’s still easy to drip or fling herbicide into the ground.  In degraded areas, if you use a sprayer for basal bark, you will have dead zones due to the small diameter of dogwood stems, so it isn’t recommended.  

·      It is unknown if any of the above methods are better than the other or whether dormant season basal bark or cut stump is worse than growing season.  The disadvantage of growing season is most stewards are swamped with weed work & will have little time for woody plant control. 

·      Overall conclusion

Herbicides are probably a necessary part of dogwood management.  Multiple application methods & how those interact with burning make it difficult to choose a path forward.  The most appealing approach to me for higher quality areas is a carefully executed single dormant season cut stump or basal bark application to the entire clone, followed by a growing season basal bark application to resprouts or cutting resprouts at the time of minimum root NSC without herbicide.  For herbiciding, 4 days precip free after treatment (maybe less for cut stump), no snow on the ground.  In remnants, the same approach except omit growing season basal bark.  For degraded areas, I think the best approach is to mow or cut when root NSC is minimum & foliar spray the resprouts.  All treatments to be done at least 3 consecutive years.  I think this strikes a decent balance between reasonable chances to be effective while minimizing labor time.

 

Can drought help?

Studies report dogwood is unlikely to be negatively affected by drought, unless it’s very severe.

 

Early invasions

If a new dogwood population appears in a prairie, it’s imperative to control it while the root is small enough to have limited NSC reserves.  Stephen Packard reports basal bark or cut stump treatment is effective on small, sparse populations.  The best method for a degraded area would be foliar spray (if no desirable natives present) or basal bark timed to coincide with minimum root NSC.  For remnants, dormant season basal bark (4 days precip free after treatment, no snow on the ground) or growing season cutting at ground level timed to coincide with NSC minimum.

 

Experiments

We need more long term experiments for dogwood control.  They should be done on obviously isolated clones. Nearby clones could be connected underground by grafting or rhizomes & if so, those untreated stems will send resources to resprouts in the treated area, making the results worse than if the entire clonal system was treated.  Choose reasonably mature clones with larger root systems (stem height >3’ in clone center).  A small monitoring plot should be installed in both the center & near the edge of the clone.  Before treatment & yearly thereafter, take a picture in the growing season (to show % leaf cover) & measure average stem height, diameter & density.  Results should not be reported until a burn has occurred & one year has elapsed since the last treatment.  This is because above ground visuals don’t tell us if the root is dead or injured & in the latter case, resprouts may be delayed a year.  Since it’s likely impossible to eliminate established dogwood, monitoring will tell us if treatment is trending toward a realistic goal of scattered small stems that can be maintained by 2-3 year fire intervals.  More experiments using glyphosate instead of triclopyr for cut stump are needed.  Hard water antagonizes glyphosate, so before using it with such water, order a Hach 5B hardness test (test strips may not have enough resolution).  One study found a large decrease in glyphosate effectiveness at 50 ppm or mg/L.  Test yearly.

 

For the remnant where I performed dormant season cut stump 2 years ago, I’m going to hedge my bets & try 2 different treatments.  I’ll try cutting the resprouts this summer in half the area & dormant season basal bark in the other half, keeping in mind there was a full season of resprouting last summer (I was too busy to treat it & didn’t have a plan), which likely replenished root carbs.  I performed cut stump herbiciding on a clone in the same area in fall of 2024 & installed 2 small monitoring plots.  No burn is scheduled for 2025, so this will better indicate if resprouts are less in the clone center compared to the edges.  I’ll perform annual summer cutting on this clone to see if one cut stump treatment followed by annual summer cutting is effective.  I’ll also try dormant season cut stump treatment using 50% Roundup equivalent in softened water on an isolated clone.  Hopefully, I can perform a summer cutting only experiment on an obviously isolated clone.  I’d like to find a method that minimizes multiple herbicide events, putting the herbicide event in the non-busy dormant season & cutting once in the busy summer weed season with motorized equipment, which goes quickly.  Since there are many variables site to site, the more stewards experimenting, the better.