Grow 503: Blue spruce, prune, indoor hibiscus

Late Winter!

It's a Big Job to Keep a Blue Spruce Small


Dear Janet,

We recently moved to a lovely condo. Our units have what looks like blue spruce between the entrances. The units were built in 1993, so these evergreens would be that age or a little more. Ours is probably 25 feet tall.

Although it looks beautiful, we're wondering if we should have it trimmed, if we could do it ourselves, how to do it and when.


Dear G.P.,

Any plant, even spruce, can be kept smaller than its potential. I've made a study of this kind of pruning.

Such pruning starts before the plant becomes too big, is done annually or bi-annually and can continue for centuries. For spruce, arb, juniper and pine, which do not normally sprout from leafless wood, starting small and cutting regularly is especially important. That's because once those branches grow for three years beyond the limits we set, pruning may leave woody stubs that can't produce new foliage and may never be completely hidden by growth from other limbs.

A healthy spruce may add 18 inches to each branch every year and reach over 60 feet in height. So to keep yours at its current size, reduce its height and width every year by up to 18 inches -- whatever its growth rate dictates. I do this for several spruces in clients' gardens and have found, as my European and Canadian mentors have before me, that this kind of pruning is best done in August when the cuts will not stimulate as much new growth.

The spruces I keep small are a size I call "Janet-plus-ladder." I don't recommend that you tackle annual pruning of a 25-foot spruce. Hire an arborist with a bucket truck or scaffolding. You'll have to do some calling and interviewing to find the right person, as this kind of pruning is not standard practice for most U.S. arborists.


Dear Janet,

I have an indoor Hibiscus, 3 or 4 years old. It lives in a sun room with high light intensity (when the sun shines). Last year it bloomed almost continuously from about February to July. Since then, one branch has grown upward to an uncontrollable height, four feet or so. The other branches remain more normal height, 15 inches. It continues to form new leaves but no blossom buds even though I have been feeding it a "bloom plus" fertilizer (10-60-10).

Has it reverted to a wild state? Should I trim it back to its original 15 inch height or would that kill it?


Dear D.L.,

If it's not being cultivated -- grown in a regulated way -- then it is indeed wild. Decide what you want from this plant and prune accordingly. If you prefer it two feet tall, don't wait for a branch to reach four feet before you cut. Clip it as soon as it crosses the bounds you set.

Routine pruning doesn't kill plants. Catch-up pruning -- what you'll do now -- won't hurt either, so long as it doesn't remove more than about a third of the plant's foliage at one time. If it will remove more than that, provide supplemental lighting to help the plant maintain vigor while it replaces leaves. Or wait to cut in late March so the plant recovers during the spring boom in sunlight duration and intensity.

Keeping the highest point of a plant clipped back tends to promote branching just below that point. So clip wayward growth at a point where new branches are acceptable -- you may have to cut to less than 15 inches to encourage branching within your framework rather than at its outer edges.

Indoor hibiscuses grown without supplemental light often stop flowering in winter. There is hardly enough sun during a Michigan winter to keep a person healthy, let alone promote flowering in a full sun plant like hibiscus. When the days lengthen, it should resume blooming.

Put away that fertilizer unless the plant is forming flower buds. Fertilizer doesn't make a plant bloom or grow. Fertilizers are to plants as vitamins are to people. Vitamins can round out the nutritional needs of an otherwise well-fed person and might even be critical for an actively-growing child but they can't do what protein and carbohydrates do, which is to stimulate or fuel growth. Plants use sunlight to make their own carbs and proteins from water and air. We apply fertilizer to supplement growth spurts.

High phosphorus fertilizers -- those with the middle number highest -- might help a plant in phosphorus-deficient soil to ripen a heavy crop of flower buds or fruit but can't create buds. Most of the time these fertilizers are a waste. They also produce high-phosphate run-off water that often reaches streams and lakes as an algae-promoting pollutant.


Green thumbs up

to an indoor bay tree and a local skunk, unlikely partners in renewing my winter-gray spirits. The one has broken bud after a winter rest, the other has begun to step out and break wind. Both tell me that spring is near.


Green thumbs down

to those who refuse to see silver linings. It is creepy to deal with certain bugs, terrible when a tree dies and frustrating to keep replanting difficult spots. But opportunity is there, too, in food for birds and chances to try new things.


Originally published 2/15/03