HG: Homegrown Column December 27, 2008

When is the best time to cut back my forsythia, before or after it flowers?
— David

Well, the “book” says to wait until after they’re done blooming to prune all spring-flowering
shrubs such as forsythia (as well as lilacs, snowball bushes and others).

These plants bloom from flower buds that were formed the prior year. Pruning them back after they flower allows you to enjoy those blooms while giving the plant enough time to push out new growth and set flower buds for the following spring.

As usual, I’m a bit contrary to these things.

Personally, I like to cut spring flowering shrubs back in early spring (March, usually). Sure, I lose the flowers from those stems I cut off, but in my experience the plant responds better and that’s the priority for me.

I’m cutting the plant back to reinvigorate it, and pruning early works better from the long-term goal of how the plant responds rather than the short-term gain of a few more flowers for one spring.

Whenever you decide to do it, this type of plant is best pruned by selecting a few of the biggest and oldest stems and cutting them off near ground level, leaving smaller stems to grow and fill the space you’ve created.

Try to avoid the temptation to shear the plant back. You can get away doing this for a while with forsythia since it has flowers all up and down the stem (doing it on lilacs or snowballs will result in no blooms since their flower buds are only at the ends of the branches), but eventually the plant will lose vigor and stop blooming well.

Doing this type of rejuvenation pruning keeps the plant fresh and vigorous for decades while preserving its natural form. You don’t have to prune these plants every year. Take a look at them every year to see if there are stems that should be removed, but I’d guess that I prune the lilacs in my yard every three or four years on average.

I read in The Daily Sentinel that you are promoting a “Scotch broom” as an ornamental shrub. I spent several years in the Pacific Northwest, where we consider this a “weed.” We certainly don’t need another noxious weed in the valley.
— Bob

You bring up an excellent point about invasive plants.

There are a number of plants out there that were introduced with the best of intentions but escaped our neatly proscribed yards and farms to invade the surrounding area, often crowding out existing native plants.

There’s been a LOT of discussion within the green industry about how to combat this issue.

The problem is that there’s no one list that works throughout North America.

For a plant to become invasive, it must thrive (even over-thrive) in that area.

Scotch broom is a terrible nuisance in the Pacific Northwest. You can see it all up and down the highways, choking out native plants. That’s not the case here in western Colorado, where our climate and soils are much different from that region.

To look at it from another perspective, I talked to a friend who has a garden center in Washington several years ago, and he sells tamarisk as an ornamental. Here it’s the plague that ate the desert Southwest.

Russian olive is also commonly sold in other parts of the country but is invasive here.

There are dozens of examples:

Maiden grass has shown some invasive qualities in the South, but it’s grown in New England where Norway maple is a big problem (along with Japanese barberry and burning bush).

English ivy is another big problem in the Pacific Northwest, but it’s planted by the hundreds of thousands as a ground cover in California.

Invasives are a regional issue. What’s invasive here may not be a problem somewhere else.

That’s why invasives are dealt with on a state or regional basis.

The state of Colorado regulates these plants through the Department of Agriculture and the State Noxious Weed Advisory Committee. Plus, most (if not all — I’m not sure about that) of the counties in Colorado have weed-control programs to deal with problems locally.

Scotch broom has been grown in this area for almost 20 years and has shown absolutely no inclination to invade the natural beauty that is western Colorado.

Believe me when I tell you that the last thing I want to do is to introduce something invasive to this area. I work to stay up to date on topics and research regarding invasives.

Dennis Hill is the nursery manager at Bookcliff Gardens, online at http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or e-mail .(JavaScript must be enabled to view this email address).

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