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Back garden buddleja - how far from a wall or foundation should butterfly bushes be?

#1 · 26th May 2025, 10:19 pm

Hi there

I have a large buddleja in my back garden. It's about 3.5m from the back wall of my house and even closer to the extension and a manhole between the plant and the extension. I love the way it looks and how many butterflies it attracts, but I recently read that these shrubs shouldn't be planted near to houses - the RHS page on these plants disagrees with this, but other sites state that these plants should be avoided. Which is correct (i.e. should I be removing this or leaving this put)? In case helpful, the shrub is so large that at its highest point, it reaches the second storey of my house.

Any guidance would be appreciated!

Thanks

#2 · 27th May 2025, 12:52 pm

Quote from Lee Garden Ninja on 27th May 2025, 12:52 pm
Hi @r_rebecca

Thanks for asking how far away from walls and houses a Buddleja should be planted so it's safe and not a pain.

The quick answer is it all depends on if you're pruning it properly every year or two. Hearing the height of it, I'd guess it's just been left to grow. This is when Buddleja become a potential problem.

I prune my Buddleja every 2 years to keep them at a max of 1.5m tall and that also helps curtail the root spread too, if not they grow huge! I can't give exact advice as I can't see your Buddleja without pictures, but I would hard prune it back in half next spring. Check the walls and flags to see if there is any obvious damage. I wouldn't get rid of it unless it's becoming a problem, but do keep it in check.

You're right that they are amazing for wildlife like butterflies and honeybees, but you'll actually get more flowers by hard pruning every 2 years, believe it or not!

How Far From Walls and Foundations Should You Plant Buddleja (Butterfly Bush)?

Plant Buddleja at least 2 metres (5 to 6.5 feet) away from walls, buildings, or foundations, especially if left unpruned. You can plant the buddleja closer to fence panels, as these only have concrete vertical supports, if you're going to hard prune the buddleja every year. The RHS is correct that they don't pose a risk if managed, but because they self-seed, if small Buddlejas take seed and root in cracks in buildings and walls, they will eventually blow they apart with their vigoorioug roots.

1. Root Spread and Foundation Risk:
While Buddleja isn’t as aggressive as plants like bamboo or willow, it does have a fast-growing root system. It’s not typically destructive, but planting it too close to structures can cause issues:

Roots can interfere with shallow foundations, especially with older buildings.

Soil near walls can be too dry, which makes root establishment more difficult and reduces plant health.

2. Airflow and Maintenance Access:
When planted right up against a wall, air circulation suffers, increasing the risk of mildew, dieback, and poor flowering. It also makes it difficult to access the plant for pruning or inspect the wall behind it.

3. Wall Reflection and Heat Stress:
Walls, particularly south-facing ones, reflect heat and light, which can stress Buddleja. This often leads to leaf scorch and early flowering. Giving the plant room helps buffer against these extremes.

4. Growth Habit:
Buddlejas are vigorous growers. They can easily reach 2 to 3 metres (6 to 10 feet) in a single season. Planting too close to a wall risks them growing awkwardly, bending away from the structure or pressing against it.

Why You Should Prune Buddleja Hard Every Year

Pruning Buddleja hard every spring is essential if you want a healthy, compact, and floriferous shrub. Here’s why this isn’t optional.

1. Prevents Woody, Leggy Growth
Without pruning, Buddleja quickly becomes top-heavy and leggy, with bare stems and flowers only at the top. Hard pruning encourages new shoots from the base, giving a bushier, more manageable shape.

2. More Flowers, Lower Down
Buddleja flowers on the current season’s growth, not old wood. By cutting it back hard, you stimulate lots of fresh shoots that produce large, fragrant flower spikes at a height you can enjoy.

3. Stops Self-Seeding Mayhem
Left unpruned, Buddleja can self-seed aggressively, especially in urban environments. Pruning hard — and deadheading — reduces seed formation and helps prevent it spreading where it’s not wanted.

4. Makes It Easier to Manage Long-Term
Annual pruning keeps the plant young, productive, and under control. Skipping years often results in a tall, unmanageable shrub that needs drastic rejuvenation — which can stress or even kill the plant.

How to Prune Buddleja Properly

When:
Late winter to early spring (February to early April in the UK), after the risk of hard frost has passed.

How Much:
Cut the stems back hard to 30–60 cm (1–2 feet) above ground level. Always prune just above a strong pair of buds.

Tools:
Use sharp loppers or a pruning saw for thicker stems. Wear gloves to protect your hands from brittle or splintery branches.

Common Mistakes to Avoid

Don’t prune in autumn — this removes the new growth that would flower the next year and can expose the plant to frost damage.

Don’t leave stems too long — you’ll end up with leggy growth and top-heavy flowering.

Don’t let it grow unchecked — Buddleja is not a tree. It needs regular, controlled pruning to look its best.

I hope that helps you!

All the best

Lee Garden Ninja

Hi @r_rebecca

Thanks for asking how far away from walls and houses a Buddleja should be planted so it's safe and not a pain.

The quick answer is it all depends on if you're pruning it properly every year or two. Hearing the height of it, I'd guess it's just been left to grow. This is when Buddleja become a potential problem.

I prune my Buddleja every 2 years to keep them at a max of 1.5m tall and that also helps curtail the root spread too, if not they grow huge! I can't give exact advice as I can't see your Buddleja without pictures, but I would hard prune it back in half next spring. Check the walls and flags to see if there is any obvious damage. I wouldn't get rid of it unless it's becoming a problem, but do keep it in check.

You're right that they are amazing for wildlife like butterflies and honeybees, but you'll actually get more flowers by hard pruning every 2 years, believe it or not!

How Far From Walls and Foundations Should You Plant Buddleja (Butterfly Bush)?

Plant Buddleja at least 2 metres (5 to 6.5 feet) away from walls, buildings, or foundations, especially if left unpruned. You can plant the buddleja closer to fence panels, as these only have concrete vertical supports, if you're going to hard prune the buddleja every year. The RHS is correct that they don't pose a risk if managed, but because they self-seed, if small Buddlejas take seed and root in cracks in buildings and walls, they will eventually blow they apart with their vigoorioug roots.

1. Root Spread and Foundation Risk:
While Buddleja isn’t as aggressive as plants like bamboo or willow, it does have a fast-growing root system. It’s not typically destructive, but planting it too close to structures can cause issues:

Roots can interfere with shallow foundations, especially with older buildings.
Soil near walls can be too dry, which makes root establishment more difficult and reduces plant health.

2. Airflow and Maintenance Access:
When planted right up against a wall, air circulation suffers, increasing the risk of mildew, dieback, and poor flowering. It also makes it difficult to access the plant for pruning or inspect the wall behind it.

3. Wall Reflection and Heat Stress:
Walls, particularly south-facing ones, reflect heat and light, which can stress Buddleja. This often leads to leaf scorch and early flowering. Giving the plant room helps buffer against these extremes.

4. Growth Habit:
Buddlejas are vigorous growers. They can easily reach 2 to 3 metres (6 to 10 feet) in a single season. Planting too close to a wall risks them growing awkwardly, bending away from the structure or pressing against it.

Why You Should Prune Buddleja Hard Every Year

Pruning Buddleja hard every spring is essential if you want a healthy, compact, and floriferous shrub. Here’s why this isn’t optional.

1. Prevents Woody, Leggy Growth
Without pruning, Buddleja quickly becomes top-heavy and leggy, with bare stems and flowers only at the top. Hard pruning encourages new shoots from the base, giving a bushier, more manageable shape.

2. More Flowers, Lower Down
Buddleja flowers on the current season’s growth, not old wood. By cutting it back hard, you stimulate lots of fresh shoots that produce large, fragrant flower spikes at a height you can enjoy.

3. Stops Self-Seeding Mayhem
Left unpruned, Buddleja can self-seed aggressively, especially in urban environments. Pruning hard — and deadheading — reduces seed formation and helps prevent it spreading where it’s not wanted.

4. Makes It Easier to Manage Long-Term
Annual pruning keeps the plant young, productive, and under control. Skipping years often results in a tall, unmanageable shrub that needs drastic rejuvenation — which can stress or even kill the plant.

How to Prune Buddleja Properly

When:
Late winter to early spring (February to early April in the UK), after the risk of hard frost has passed.

How Much:
Cut the stems back hard to 30–60 cm (1–2 feet) above ground level. Always prune just above a strong pair of buds.

Tools:
Use sharp loppers or a pruning saw for thicker stems. Wear gloves to protect your hands from brittle or splintery branches.

Common Mistakes to Avoid

Don’t prune in autumn — this removes the new growth that would flower the next year and can expose the plant to frost damage.
Don’t leave stems too long — you’ll end up with leggy growth and top-heavy flowering.
Don’t let it grow unchecked — Buddleja is not a tree. It needs regular, controlled pruning to look its best.

I hope that helps you!

All the best

Lee Garden Ninja

#3 · 29th May 2025, 12:31 am

Thanks very much, Lee. Really helpful!

Could I just check whether pruning around this time of year would be a bad idea? Asking as: it's so big that I'm conscious that waiting until next April to hard prune could let it get out of control; and your advice seems to suggest that pruning in autumn is the big no-no.

I would've attached some photos but couldn't get them to upload I'm afraid!