Black Knight Butterfly Bush (Buddleja Davidii) Plant Care Guide

Last Updated on October 31, 2021 by Phil

The black knight butterfly bush (Buddleja davidii ‘Black Knight’) is a beautiful fragrant flowering shrub that’s great for adding color to your garden. It likewise attracts butterflies and hummingbirds to your yard.

The plant gets its name from its dark, purple color, which comes to full bloom during the summertime. It grows up to between 5 to 8 feet tall and about 4 to 6 feet wide making it a good choice if you need border plants for your garden.

That said, you can likewise grow it in containers. Although, more often than not, it is grown in cut flower gardens.

Native to China and Japan, the plant likes bright light and can tolerate cold temperatures.

Table of Contents

Black Knight Butterfly Bush Plant Care

Black Knight Butterfly Bush Light

The black knight butterfly bush needs full sun. As such, you want to provide it with at least 6 to 8 hours of sunlight on a daily basis. In warm conditions, it will be fine with 6 hours or so. But, in most cases, the more bright sunlight it receives the happier it will be.

If you don’t give it enough light, you’ll start noticing it produce less flowers. And, as it becomes sparse it will look weak and scrawny. Thus, it is not a good idea to leave it somewhere there is a shade.

The best place to put this plant in your garden is facing south. This gives it a lot of sunlight. And, because it doesn’t mind staying under direct sunlight, you won’t have to worry about it experiencing sunburn like other plants.

East and west facing gardens will work as well. But, you need to make sure it receives enough light throughout the day.

The last place you want to put it is somewhere facing north. The lack of sunlight here will be problematic for the black knight butterfly bush.

Black Knight Butterfly Bush Temperature & Humidity

The black knight butterfly bush is hardy to USDA zones 5 to 9. This means its is able to tolerate colder climates down to -15 to -20 degrees Fahrenheit. Although, in the colder regions (zones 5 and 6), its leaves will drop and the plant will go dormant in the winter to survive the lower temperatures. Then, regrow once springtime arrives.

Many farmers will also cut the plant close down to the ground during winter for this purpose because the plant grows more vigorously and produces more flowers the next season.

Watering Black Knight Butterfly Bush

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The black knight butterfly bush is drought tolerant. Thus, you it doesn’t need a ton of watering. As long as it receives an inch of water each week, it will be happy.

This makes a rain gauge a good investment. The device is cheap and it will allow you to easily monitor exactly how much rainfall there is recently. This allows you to back off from watering if there’s enough rainfall. And, only supplement watering if it hasn’t rained in the last 7 days.

This is important if you want to keep your black knight butterfly bush healthy because it doesn’t like wet or soggy conditions.

In its lifetime, there are only two instances where you’ll need to increase watering for this plant.

During its first year. This allows the plant to establish a strong, deep root system. After year 1 when the plant has established itself, you can scale down its moisture requirements to that mentioned above.
During hot summers. Increase the frequency of watering when summers get hotter than usual. A lot of this will depend on how warm the temperature gets on that particular year.

Outside of these two situations, the black knight butterfly bush doesn’t need more than occasional watering.

That said, you can use mulch and compost to help the plant retain water. Doing so also prevents weeds from growing.

Soil

From the section above, you can already guess that the black knight butterfly bush needs well-draining soil. This is its concern when it comes to other.

Other than that, it won’t really care what kind of soil it gets as it can adapt to different conditions. However, its biggest weakness is staying wet.

That said, there are a few little things you can do to tweak the soil to allow your butterfly bush to grow optimally. These are.

Fertile soil. Soil rich in organic matter will help it grow fuller and bushier.

Slightly acidic soil. Keeping the soil pH at between 6.0 to 7.5 allows the plant to produce more vibrant color.

Caution: The Plant is Invasive

In most parts of the U.S., the butterfly bush has been classified as an invasive plant. This means that the plant, when left alone to grow, has the tendency to crowd and take over the spaces of nearby plants including those native to the area that are essential for wildlife survival.

This is especially true in warmer climates, where the it spreads aggressively as a weed. The case is less so in cooler climates when it can be contained as long as you deadhead the flowers once they begin to fade.

Thus, depending on where you live, you will need to decide whether you want to grow this plant or not. Some gardeners choose to replace them with more sterile plants to avoid butterfly bush’s self-reseeding problem.

Others, are less concerned and are happy with the plant. In these cases, they make sure to deadhead it to keep it from spreading its seeds aggressively across their plot.

Fertilizing

The black knight butterfly bush is a resilient shrub that doesn’t require a lot of human intervention to grow. In fact, it doesn’t need fertilizing at all, if any. The plant will happily grow to be healthy even if you don’t feed it.

This is true even in average soil. Its only requirement is that the soil drains excess moisture well. If you give it that, it will bloom and grow without you having to spend on fertilizer.

That said, there are a few circumstances where you may need to supplement with fertilizer.

The plant isn’t growing as it should. If this happens, you’ll want to try to see if giving it fertilizer will help. If it doesn’t, check the other requirements we’ve listed in this article to see if anything is missing or overdone.
Poor soil. The plant grows better in fertile soil. As such, if your garden soil happens to be short on nutrients it will slow down the growth of your black knight butterfly bush. In this case, you may supplement with fertilizer.

While your first instinct may be to go out and get fertilizer for the plant, I suggest you do something else.

That is, to add organic compost to your soil first.

I feel that this is a much better long-term solution as it allows you to improve the soil with the compost. Not only is it rich in humus and nutrients, compost also helps the soil become loose and fluffier.

Over time, the soil will become more nourished. And, when this happens it will be able to “feed” the plant naturally. This allows you to skip fertilizing altogether.

However, amending soil with compost takes time to “fix” the soil. So, if you want something more immediate, you can use granular or slow release fertilizer. The best time to apply it is during the spring.

But, be aware of the black knight butterfly bush’s low requirements. Adding too much can lead to fertilizer burn.

Also, even if you don’t overfeed it, you’ll want to avoid giving it lots of plant food. The reason is that too much fertilizer will encourage its foliage to grow. In doing so, it focuses less on its flowers, which are its crowning glory.

When this happens, the plant’s beauty will be “lost”.

Last but not least, adding a healthy layer of mulch during the winter helps protect the plant’s roots from frost.

Pruning

The black knight butterfly bush is a fast growing shrub. It likewise grows to between 8 to 12 feet. Thus, pruning helps keep it under control. Doing so also helps you limit its size and manicure its shape. If you let the plant be, it will become messy and unruly.

That said, the most important reason to prune it is to contain its invasive nature.

Deadheading the flowers when the begin to fade prevents the plant from spreading is seeds. This keeps it from aggressively growing in different areas of your yard or garden without you planting them.

Additionally, trimming back the blooms helps it grow more flowering in the upcoming cycles.

The best time to prune your black knight butterfly bush is late in the fall or during spring.

How to Prune Black Knight Butterfly Bush

When pruning, treat the plant like you would a tree or shrub. Thus, besides deadheading the spent flowers, you want to trim away any dead, dying, damaged or diseased limbs.

Finally, trim as needed depending on how big you want the plant to be. Because it can grow to become fairly sizable, you’ll need to decide how much you want to prune to keep it manageable for your backyard.

Many gardeners also cut the plant down to the ground leaving only a foot. This allows it to come back more revitalized in the spring.

Propagation

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As mentioned, your black knight butterfly bush will aggressively try to reseed itself in areas of your yard and garden depending on where the wind blows these seeds. As such, deadheading is crucial if you want to prevent this from happening.

But, there may also be times where you want to add more of this dark colored butterfly bush plants to your garden.

If such is the case, you’ll be happy to know that you can propagate it via seeds, division and cuttings.

How to Propagate Black Knight Butterfly Bush via Cuttings

This is the easiest way to propagate butterfly bush. For one, it takes less work than division. And, it doesn’t take as long as propagating from seed to grow.

Start by taking a branch cutting. You want to pick one that is at least 3 to 4 inches long.
Remove the lowest leaves since they will be placed into the soil anyway.
You can dip the cut end into rooting hormone powder to speed up the rooting process.
Place the cutting into potting soil.
Water the soil regularly to keep it moist.
In a few weeks, the roots will start growing.

How to Propagate Black Knight Butterfly Bush through Division

Propagation by division takes a little more work than cuttings. But, you instantly get a semi-grown plant. This saves a lot of time since you don’t need to wait for it to root and start growing from there.

The best times to propagate via division is during the spring or fall.

First, dig up the plant.
Pick a healthy section of the plant. You will be separating this from the mother plant by tracing it down to the roots.
Use your hands to divide the selected section.
Plant the new section into the ground and place the mother plant back into its spot.

How to Propagate Black Knight Butterfly Bush from Seed

Propagating via seed takes the longest of the three methods. Here, you’ll need to chill the seeds about a month before you sow them.

When planting, lightly cover the seeds with soil. You don’t want to overdo it because butterfly bush like a lot of sunlight. And, it will need that sunlight to germinate.
From there, keep them moist.
It will take at least a few months to germinate.

Transplanting & Repotting

As mentioned, black knight butterfly bush grow to become sizable. So, you want to give them enough space between one another or other plants to grow. The space also allows air to circulate freely. In doing so, it prevents leaf spot from happening as air helps dry excess moisture on foliage.

The best time to plant your butterfly bush is during the fall or spring. When doing so, you want to get them into the ground before first frost. This gives them enough time to establish their root system before the cold arrives.

The best place to grow the plant is where it gets a lot of sunlight. It needs at least 6 to 8 hours of light daily. Plus, it doesn’t like wet, soggy soil. So, you need to make sure that the soil in that location drains well.

How to Transplant Black Knight Butterfly Bush into Your Garden

You’ll likely start with a plant that’s in a container. Thus, use the size of the container to gauge how big the hole you need to dig. Ideally, dig a hole with 2x the diameter of the pot the plant is in now.
Remove the plant from the pot.
Insert the plant into the hole. Make sure that the root ball is level to the surface of the ground.
Cover the space around with soil
Water the plant regularly. This will help the plant establish strong, deep roots. After year 1, you can reduce the amount of watering since the plant ahs already set its root system down.
It will take about 1 to 2 months for the plant to establish its roots in its new spot.

Toxicity

The plant is not toxic to people or animals. Thus, you don’t need to worry about keeping it away from children and pets. Although, as always, ingesting any plant can be a choking hazard.

Pests and Diseases

Black knight butterfly bush doesn’t come with many pest or disease issues. But, on occasion, they may experience spider mites, weevils or mullein moths. If you do notice any infestations, it is a good idea to treat them naturally.

You want to be careful with using pesticides because the plant attracts pollinators. Using pesticides will harm these creatures.

When it comes to diseases, fungal leaf spots can be a problem with too much watering. Thus, you will want to avoid soaking the plant’s leaves when watering.