Just How Invasive Is Comfrey? And How to Control It

As gardeners, we’re always warned not to allow invasive plants to take hold of our gardens. This mostly goes for weeds, but there are certain beneficial herbs and plants that spread more than we’d like. Comfrey is definitely one of them.

Despite it being invasive, people still grow comfrey. Well, there’s actually more to comfrey than meets the eye. As it turns out, this plant is the ultimate “black gold” mine for gardeners. 

In this article, we’ll discuss what to expect when growing comfrey for the first time and how you can keep it under control.

Why Gardeners Grow Comfrey

Many gardeners grow comfrey for various reasons, be it for ornamental, agricultural, or medicinal purposes.

Ornamental: A very prolific perennial, comfrey makes an excellent groundcover for improving soil condition over time. It also bears clusters of beautiful bell-shaped flowers that bees and butterflies delight in, with colours ranging from purple to a soft pink hue.

Pest & Disease: Likewise, comfrey plants are great for controlling pests and diseases in the garden. It can be used against powdery mildew or to keep garden slugs at bay. 

Food & Fertilizer: Gardeners also love growing comfrey as a valuable food source for plants and livestock. High in nutrients, comfrey works wonders as mulch, liquid fertilizer, animal feed, and as “greens” or nitrogen-rich material for compost.

Herbal Medicine: Lastly, comfrey may be grown in containers at home as a first aid. Also known as “knitbone”, it’s used in traditional medicine to treat all sorts of external injuries, including bruises, muscle sprains, burns, torn joints, and fractured bones.

What to Expect When Growing Comfrey

Gardeners who grow comfrey must be ready at all times to gamble a small corner of their yards (or sometimes, the entire plot itself) just to get a piece of that black gold. Before you go out and get one for your garden, know first that:

1. Comfrey is a highly invasive plant, especially varieties that grow from seed.

2. It is extremely difficult to get rid of once it is fully mature and well-established.

3. Sterile varieties are preferred by most gardeners compared to self-seeding types.

4. Every root fragment turns into a new plant, so be careful when cutting back.

5. Some parts of the plant may cause skin irritation (don’t forget to use gloves!)

6. This herb is said to have cancer-inducing chemicals and may cause liver damage.

As useful and edible as comfrey may seem, it’s a slightly toxic plant and therefore not recommended for daily consumption. It’s also stubbornly persistent, so watch out!

When Comfrey Grows Out of Control

Once comfrey grows out of control, it can have a devastating impact on your garden. 

Every season you’ll have to get rid of new comfrey plants, as they self-seed and sprout pretty much everywhere. They spread slowly, but they can easily take over your property as the remaining stuff is difficult to get rid of.

Not only that, but you’ll also lose precious time trying to get rid of this invasive herb whenever it comes back. Your neighbours won’t be thrilled when they have to keep mowing over the comfrey that’s suddenly growing in their front yard.

Worst of all, an invasive plant like comfrey can upset the balance of a local ecosystem.

Left in the wild, comfrey can easily compete with endemic plants. This will, in turn, affect native wildlife that depends on these plants to survive, especially endangered ones.

Where It Grows Best and Perfectly Contained

Comfrey performs best under full sun with consistent watering, but it can also withstand drought. It grows well in partial shade, but not as vigorously as those exposed to more sunlight. Each plant grows about 2-4 feet wide, often reaching a height of 3-4 feet, bearing colourful flower clusters that garden creatures love.

To limit its spread, comfrey needs to be contained permanently, as if it were spearmint. Here are some of the best places suitable for growing comfrey:

• TALL CONTAINERS
  
  Tall containers and deep planters are ideal since comfrey roots tend to grow deep, forming wide networks. Make sure to place these containers on hard surfaces such as bricks, tiles, or cement surfaces so the roots won’t be able to form new plants. Always check the drainage holes for escaping roots.
  
• ENCLOSED CORNERS
  
  Choose a permanent location in your garden if you want to grow comfrey directly on the ground. Enclosed spaces work best, especially those far from vegetables and other crops. You can keep it as an ornamental shrub, encouraging friendly pollinators to visit your garden every now and then.
  
• AROUND TREES
  
  Comfrey may also be grown around trees to serve as groundcover. Plant them about 12-15 inches away from the base and keep them regularly trimmed. You can also leave stem cuttings to wilt and become fertilizer for the trees. In the long run, comfrey keeps trees healthy while breaking down hard, compacted soil for tree roots to flourish
  
• RAISED GARDEN BEDS
  
  Add barriers to raised garden beds (such as stainless steel mesh) for drainage and to keep comfrey roots in check. If you choose to grow comfrey this way, make sure not to plant anything else next season, as any leftovers can grow back and take over new vegetable seedlings.

How to Keep Comfrey from Taking over the Garden

Aside from choosing a suitable location for growing comfrey, there are also many ways you can prevent a comfrey invasion early on.

Select a sterile variety

This is the MOST IMPORTANT factor when it comes to minimizing comfrey spread. It’s very important to know which variety you’re growing, as there are many types of comfrey plants that produce fertile seeds and others that don’t.

The Bocking 14 and Bocking 4 varieties are great examples of sterile comfrey plants. Both are Russian comfrey cultivars known for being fast growers with limited spread. They are easily propagated through root and crown divisions but never from seed.

These varieties grow tall, but the plants stay there – no matter if birds, insects, rainfall, and wind whisk their seeds all over the place, they still won’t germinate!

Know the climate in your location

Comfrey loves sunny areas with fertile soil and a good amount of rainfall. Once your plants find an open plot with these conditions, brace yourself for a comfrey invasion.

The key here is to hold back the plant’s basic needs and slow it down from forming new ones. You can do this by growing comfrey somewhere with more shade or by keeping them under trees so they don’t get too much sun or moisture.

Harvest and trim regularly

Most types of comfrey spread rapidly once they begin flowering, seeding, or growing new roots. Comfrey starts blooming late in May or early June, so keep this in mind when cutting back mature plants. Trim frequently before the flowers go to seed.

You can do this 3 to 4 times a year and trim the plants about 5-6 inches from the ground, depending on how fast they grow and spread.

Always harvest comfrey and replace mature plants with new ones. That way, you can contain the plants before those roots make it all the way to rock bottom (pun intended).

Restrict comfrey to one spot

Never plant comfrey all over the place. Instead, just keep them in a single area so they’re easier to maintain once restricted to a corner. You can also place mulch or black plastic sheet in places where you don’t want comfrey to grow. Better safe than sorry.

Most importantly, never dig around the plant (unless you want it removed), as digging encourages the plant to spread more. You never know when you’ll split those roots below, and comfrey plants are more than happy to sprout from new root pieces.

How to Get Rid of Comfrey for Good

A comfrey invasion can be so severe that it’s sometimes best to take desperate measures. Or perhaps, you just realized comfrey sucks and you’d rather get rid of this pesky herb. Either way, it takes time to completely remove comfrey from its spot.

Depending on the maturity and growth of the plants, getting rid of comfrey requires patience and persistence for any gardener.

FOR YOUNG COMFREY PLANTS AND OFFSPRING

Young comfrey plants are easier to remove since the roots haven’t reached the hard layers of the soil yet. They can be tricky to deal with, but not as long as you find every one of them. With that in mind, you should be able to do the following:

• Dig out the plants before they spread.
  
  As you dig, gently loosen the soil around the plant. Comfrey roots grow and spread to the side about 2-3 feet, so be careful not to break any roots. Then, gather every single piece that you can find. You can also use something to filter and sort through the soil, just to be sure you don’t leave any remaining parts or seedlings behind.
  
• Cover the area with a plastic sheet.
  
  Use a black plastic sheet to finish off any plants hiding below. Comfrey requires full sun to thrive, and without enough sunlight, any sprouts left behind won’t make it to maturity. This works best for newly germinated seedlings. 
  
  You can get some at your local garden center or simply purchase one online. Once the soil is clear of comfrey, you may use the plastic sheet to protect your vegetables.

FOR MATURE AND FULLY ESTABLISHED COMFREY PLANTS

Unlike their offspring, mature comfrey plants are more difficult to remove. They have large taproots that cling stubbornly to the soil and numerous flowers that contain very tiny seeds. What’s more, they grow back rapidly once harvested, especially during the blooming season. For this task, you’re going to need more tools to get the job done.

• Use a weed removal tool.
  
  A weed removal tool with high leverage would be great for uprooting mature comfrey plants. You can get one at the hardware store (your neighbor might even have one). 
  
  Again, keep your eyes peeled for any root pieces left on the ground while you’re at it. It’s best that you also remove flower stalks and any spent blooms from the uprooted comfreys (if you have self-seeding types) so the seeds won’t scatter all over the yard.
  
• Mow and mulch the area.
  
  Tired of sorting through the soil? Try mowing comfrey plants back to ground level. Afterwards, collect the cuttings and flower stalks before covering the area with mulch.
  
  Arborist wood chips would be perfect for dealing with widespread comfrey invasion. Apply a thick, even layer as mulch right after mowing. You can then rake those wood chips away once the plants die off.
  
• Prepare an organic acid solution.
  
  This is an alternative to herbicides, especially when you have other vegetables and ornamental plants growing nearby. Organic acids can damage plant cells, eventually “burning” the plants until they wilt.
  
  Create an organic acid solution using 1 quart of acetic acid (vinegar) and 4 oz of citric acid (lemon or any other citrus fruit). Mix with water to get a 5% – 30% concentration. Then, use the solution to spray all over the comfrey.

Conclusion

Having comfrey around can feel as if you’re growing and harvesting your own compost. There are so many benefits to this herb, but if you’re not careful enough, you might end up with more plants than you can handle.