Subject: Resources for your garden: Plants to avoid in New Hampshire

Dear Mary

Thank you for contacting the UNH Extension Infoline regarding your perennials issues in your garden.

Following up on your email from 4/8/2026, I’ve attached some general information to help get you started.

It sounds like you’ve done a tremendous amount of work clearing out those aggressive spreaders! New Hampshire actually has specific regulations to help gardeners avoid plants that can quickly become unmanageable or harm the local ecosystem.

We are committed to finding the right solution for your lawn and garden and are happy to provide further guidance as needed.

Please feel free to reach out with any further questions.

Best regards

Haim
Master Gardener Volunteer
UNH Extension Infoline Team

General information

.To keep your "contained" garden from being overrun again, here are two key lists to check before you buy anything new:

• NH Prohibited Invasive Species List: These are plants that are legally banned from being sold, moved, or propagated in the state because they are so destructive. Common offenders often found in older landscapes include:

Burning Bush (Euonymus alatus),Japanese Barberry (Berberis thunbergii),Norway Maple (Acer platanoides),Oriental Bittersweet (Celastrus orbiculatus),Japanese Knotweed (Reynoutria japonica)

• NH Invasive Plant "Watch List": These plants aren't illegal yet, but they show invasive tendencies in our region and are best avoided. This list includes:

Common Periwinkle (Vinca minor),Ground Ivy (Glechoma hederacea),Creeping Buttercup (Ranunculus repens)

to deal with your existing plant follow some suggestion:

Methods for Restricting Perennials:

Divide Plants: Dig up overgrown perennials (especially hostas, daylilies, and irises) in early spring or fall, splitting them into smaller pieces with a spade to prevent overcrowding and expansion.

Physical Barriers: Install plastic or metal landscape edging deep into the ground (6+ inches) around spreading plants to create a barrier.

Bury Potted Plants: Place aggressive spreaders (like mint or lemon balm) inside plastic pots with the bottoms removed or drilled, then bury the pots, leaving the rim above ground level to contain roots.

Deadheading: Cut off spent flower heads before they go to seed to prevent new plants from popping up throughout the garden.

Regular Cultivation: Frequently dig up seedlings in the spring, especially around plants known to self-sow.

Selective Pruning: Cut back spreading perennials to 6–10 inches tall in early summer to restrict height, especially for plants like Joe-pye weed or sneezeweed.

Avoid Landscape Fabric: According to the UNH Extension, avoid using landscape fabric for weed control in perennial beds, as it becomes a maintenance nightmare when weeds take root through it, making removal

Helpful Direction & Alternatives
For specific advice on what to plant instead, the UNH Extension provides a fantastic Alternatives to Invasive Landscape Plants fact sheet. They suggest beautiful, non-aggressive plants.

Helpful links

https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/resource001358_rep1815.pdf

https://extension.unh.edu/resource/alternatives-invasive-landscape-plants-fact-sheet

https://extension.unh.edu/blog/2019/05/should-i-use-landscape-fabric-keep-weeds-out-my-perennial-garden

Hi Mary,

Please see the attached list of plants that are best to avoid for your garden. Many of these are considered invasive in our area or tend to take over a landscape very quickly. 

Most local garden centers can offer great advice on what to avoid, and they generally won't sell plants that are on the official invasive species list. 

I hope this helps with your planning!

https://www.nhdfl.dncr.nh.gov/sites/g/files/ehbemt866/files/documents/nh-invasive-plant-list.pdf