Trees and Shrubs That Self-sow Excessively

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The Norway maple (Acer platanoides) produces so many seedlings it can easily become a garden pest. Source: www.massaudubon.org.

All plants reproduce. If they didn’t, they’d go extinct! But most do so modestly, producing a plant here and there, just enough to maintain their population. Others, though, do so profusely, becoming an annoyance to gardeners and spreading into the wild far from where they are native. Most such plants are simply called weeds and they tend to be annuals, perennials or biennials. However, there are also trees and shrubs that overdo it and can become invasive due to aggressive self-sowing as well.

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False spirea (Sorbaria sorbifolia) can be highly invasive locally, but tends to spread via suckers rather than seed, so doesn’t get far. Source: Fanghong, Wikimedia Commons

Of course, there are different ways in which a plant can become invasive. Through suckers, for example. Think of false spirea (Sorbaria sorbifolia) or staghorn sumac (Rhus typhina). They certainly get around, but since they sprout from wandering roots, they only tend to be invasive on a very local scale. Woody plants that spread by seed can get much farther. What with birds, squirrels and wind to carry them greater distances, they can really get around.

I, for example, have no Norway maple on my property, nor do any of my immediate neighbors, but there are several further down the street and as a result, I find hundreds of Norway maple seedlings in my gardens every year.

The Ones That Overdo It

What follows is a list of trees and shrubs that have the reputation of being invasive through their seeds, but…

Not all plants on the list will be invasive under all conditions. They’ll only cause problems when the local environment is appropriate to their needs. For example, tamarisk or saltcedar (Tamarix ramosissima) is very invasive in drier climates, but not a problem at all in more humid ones, while Japanese barberry (Berberis thunbergii), which can be terribly invasive in moderate climates, doesn’t produce fertile seed in colder ones and thus is not a problem there.

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Frangula alnus ‘Ron Williams’ Fine Line is a sterile columnar form of the otherwise invasive alder buckthorn (F. alnus). Source: springmeadownursery.com

Also, there are sterile forms of many of the trees and shrubs listed here, such as Frangula alnus ‘Ron Williams’ Fine Line, a sterile form of the otherwise invasive alder buckthorn (F. alnus), while new sterile varieties of Japanese barberry (Berberis thunbergii) and winged euonymus (Euonymus alatus) are either under development or being introduced. If there’s a tree or shrub you like, but it has invasive tendencies, you can often find a sterile or nearly sterile form you can use with impunity.

Mulching Can Help

Fortunately, using a good mulch will prevent most tree and shrub seeds from germinating. They simply can’t germinate through a thick mulch. That said, some trees—especially nut trees—are among the few plants whose extra robust seeds really can germinate through a mulch. Those that are invasive even under 2 inches (5 cm) of mulch are marked with an asterisk (*).

You Choose

I’m not saying do not plant the plants listed here—some are great garden plants!—but forewarned is forearmed!

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Himalayan blackberry (Rubus armeniacus, syn. R. discolor, zone 7) is extremely invasive in some climates, but not hardy enough to be a problem in colder areas. Source: www.nwcb.wa.gov

  1. Alder buckthorn (Frangula alnus, formerly Rhamnus frangula) zone 3
  2. American elm (Ulmus americana) zone 3
  3. Amur honeysuckle (Lonicera maackii) zone 2b
  4. Ash (Fraxinus spp.) zone 2 to 7, according to species
  5. Autumn olive (Elaeagnus umbellata) zone 4
  6. Bird cherry (Prunus padus) zone 2
  7. Black locust (Robinia pseudoacacia) zone 4b
  8. Box elder* (Acer negundo) zone 2
  9. Blackberry (Rubus spp.) zone 2 to 8, according to species
  10. Bramble (Rubus spp.) zone 2 to 8, according to species

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    Renowned for its stunning fall colours, burning bush (Euonymus alatus) can nevertheless be invasive in some areas. Source: www.plantes-shopping.fr

  11. Burning bush (Euonymus alatus) zone 5
  12. Butterfly bush (Buddleia davidii) zone 6b
  13. Chinese elm (Ulmus pumila) zone 2
  14. Chokecherry (Prunus virginiana) zone 2b
  15. Common buckthorn (Rhamnus cathartica) zone 2
  16. Dog rose (Rosa canina) zone 4
  17. European birch (Betula pendula) zone 3
  18. European privet (Ligustrum vulgare) zone 4
  19. Guelder-rose (Viburnum opulus) zone 3
  20. Horse chestnut* (Aesculus hippocastanum) zone 4b
  21. Japanese barberry (Berberis thunbergii) zone 4
  22. Morrow’s honeysuckle (Lonicera morrowii) zone 4
  23. Mountain ash (Sorbus spp.) Zone 3
  24. Multiflora rose (Rosa multiflora) zone 5b
  25. Norway maple* (Acer platanoides) zone 4b
  26. Norway spruce (Picea abies) zone 2b
  27. Pin cherry (Prunus pensylvanica) zone 2

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    Common buckthorn (Rhamnus cathartica) is very invasive in most temperate climates. Source: nyc.books.plantsofsuburbia.com

  28. Plane (Platanus spp.) zone 5 to 9, according to species
  29. Raspberry (Rubus idaeus) zone 3
  30. Redleaf rose (Rosa glauca, formerly R. rubrifolia) zone 2
  31. Rose of Sharon (Hibiscus syriacus) zone 6b
  32. Rowan tree (Sorbus spp.) Zone 3
  33. Rugosa rose (Rosa rugosa) zone 3
  34. Russian olive (Elaeagnus angustifolia) zone 2b
  35. Saltcedar (Tamarix spp.) zone 5 to 9, according to species
  36. Scots pine, (Pinus sylvestris) zone 2b
  37. Sea buckthorn (Hippophae rhamnoides) zone 2b
  38. Siberian elm (Ulmus pumila) zone 2
  39. Siberian peashrub (Caragana arborescens) zone 2
  40. Silver maple* (Acer saccharinum) zone 2
  41. Small-leaved lime* (Tilia cordata) zone 3
  42. Sugar maple* (Acer saccharum) zone 4
  43. Tamarisk (Tamarix spp.) Zone 5 to 9, according to species

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    Tatarian honeysuckle (Lonicera tatarica) can be quite invasive in temperate climates. Source: www.florafinder.com

  44. Tatarian honeysuckle (Lonicera tatarica) zone 4
  45. Tree of heaven (Ailanthus altissima) zone 6b
  46. Walnut* (Juglans spp.) Zones 4b to 8, according to species
  47. Wayfaring tree (Viburnum lantana) zone 2b
  48. Winged euonymus (Euonymus alata) zone 5

A Hedge for Laidback Gardeners

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Get a life: stop pruning that formal hedge and put in an informal one!

For many people, a hedge has to necessarily be neatly trimmed… and that involves a lot of maintenance. But that’s only one kind of hedge: the “formal” hedge. If you want to delineate your property without having to prune multiple times each summer, why not consider growing the formal hedge’s laidback cousin, the informal hedge?

It’s certainly simple enough to do. Just plant shrubs in a row and let them grow, period.

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Most flowering shrubs, like this Spiraea x cinerea ‘Grefsheim’, bloom much more abundantly when used as an informal hedge.

The hedge will then take the form of the shrub that composes it. If the shrub is naturally rounded, the hedge will be rounded, if it tends to be more upright, the hedge will be upright. If the shrub arches outward, so will the hedge.

As for maintenance, there is almost none involved… if you choose low-care shrubs!

Making the Right Choice

When choosing a shrub for an informal hedge, here are a few questions to ask:

  1. What is its maximum height and spread?
  2. Do its needs (exposition, soil type, drainage, hardiness zone, etc.) match my growing conditions?
  3. Is it available at a price that suits my budget?
  4. Is it naturally resistant to pests and diseases?
  5. Is it reputed for its ability to grow with little to no care?

Your local garden center operator ought to be able to answer many of these questions for you: don’t hesitate to ask.

Good Shrubs for an Informal Hedge

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Frangula alnus ‘Ron Williams’, sold under the commercial names Fine Line® and Straight Line®, makes one of the narrowest informal hedges.

Here are some shrubs that are good choices for an informal hedge in temperate climates (if your climate is tropical or subtropical, the choice is much greater!).

Note that there are multiple cultivars most of most species mentioned. They differ notably in their height and diameter, their hardiness, and the color of their flowers, fruits, and leaves.

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The rugosa rose (Rosa rugosa) makes a very hardy, very tough hedge… for areas that don’t suffer from rose chafer or Japanese beetle!

Also, the hardiness zone given is the coldest one the shrub can take. If there is a range of zones shown (example, 4-9), that means the species available vary in their degree of winter hardiness.

  1. Alpine currant (Ribes alpinum) zone 4b
  2. Amur Maple (Acer tataricum ginnala) zone 2a
  3. Boxwood (Buxus spp.) zones 4-9
  4. Fernleaf buckthorn (Frangula alnus cvs, syn. Rhamnus frangula) zone 3b
  5. Hedge cotoneaster (Cotoneaster lucidus) zone 2b
  6. Honeysuckle* (Lonicera spp.) zone 2
  7. Japanese barberry (Berberis thunbergii) zone 3

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    Informal hedge of Japanese barberry.

  8. Ninebark (Physocarpus spp.) zone 2b
  9. Privet (Ligustrum spp.) zones 4-9
  10. Purpleosier willow, Arctic willow (Salix purpurea ‘Nana’) zone 2
  11. Shrub roses (Rosa spp.) zones 2-7
  12. Shrubby cinquefoil (Potentilla fruticosa) zone 2
  13. Siberian peashrub (Caragana arborescens) zone 2
  14. Snowberry (Symphoricarpos spp.) zones 2-5
  15. Spirea (Spiraea spp.) zones 2-6
  16. Viburnum (Viburnum spp.) zones 1-7

* Avoid honeysuckle varieties that are subject to witches’ broom.

Avoid Conifers

You will notice the absence of conifers among the selections of shrubs for an informal hedge. That’s because conifers continue to grow throughout their lives. To stay within the acceptable size limits of a hedge, they all need regular trimming, at least once they’ve reached the desired maximum size. This makes them poor choices for an informal hedge.

Mixed Hedge

20160629E.jpgA successful informal hedge can be composed of a single species of shrub, but this does leave it open to possible problems. In fact, you’ve just planted a monoculture: if an insect or disease appears that likes that particular species or cultivar, you’ll have quite a problem!

To avoid this, why not plant a mixed hedge? There is nothing that prevents you combining two or more varieties of shrubs in the same hedge.

If you’re open to this idea, you might want to include shrubs that extend the flowering season by blooming at different times, or plant a variety of berry-producing shrubs that ripen at different seasons, allowing you to attract fruit-eating birds for much of the year.


Enjoy your laidback hedge!

Spiraea cinerea Grefsheim