Growing Concerns 602: Arborvitaes, root bound


Evergreen hedge is the garden's backbone and worth fighting for

Dear Janet,

The death of my eight arborvitaes would be devastating to my little garden. It's a screen from close neighbors and it directs the eye out to the park behind us, a view that is lovely.

In 1995 my husband and son dug holes in our clay soil just about the size of the root balls of the arborvitae. We added bone meal and blood meal to the bottom, unwrapped the root balls and dropped them in. I know we should have dug wider holes but the clay was daunting. They have been healthy all these years and are about ten feet tall now.

Last summer I noticed some edges near the middle to the bottom of three of the shrubs that were covered with a shiny black film that wouldn't wash off. So I trimmed it all off. It was only on a small portion of those shrubs.

I then sprayed them all with insecticidal soap and gave them their usual Miracid treatment.

But throughout the summer I could see them turning more and more brown in big patches, especially the inside. No more black stuff showed up.

This fall I sprayed them with anti-dessicant for protection.

My only opinion is that maybe the roots have gotten so big that they may be pushing into the clay soil and not getting to develop well. What do you think of this theory?

This spring I will aerate around the extended root system on my side with a large fork tool I bought. (My neighbor's concrete butts up to their other side.) That's the only other idea I have had.


Dear B.G.,

Your opinion and plan are both sound. You're doing the two most important things, which are to keep a close eye on your plants, look into problems right away, and always begin with questions about the site and general vigor.

The black film and browning are probably not directly related to one another but together they're a message that the plants are weakening for some reason. It's entirely possible that they've become root bound. Although you're late in following up those too-small planting holes with aeration, it's better now than never. When we plant in hard packed soil it's vital to aerate each year in a gradually widening zone around those plants.

Dig in this spring to confirm your theory. See if roots have begun to turn and stack up at the concrete edge. Look along your side to see if the roots ever left the original planting holes.

Don't despair over confined roots. Coax them out with aeration. Also, take heart in knowing it's possible to grow almost anything in a pot, without up-potting, with careful watering and fertilizing. You can keep these shrubs going the same way.

If their roots have not spread wide, lay out weeper hose along the edge of the root zone. Let those hoses drip very slowly from mid-April right through into fall.

Continue fertilizing regularly during spring and early summer. Don't be stingy. If your eight shrubs' roots cover about 100 square feet -- a space 25 feet long and four feet wide -- then you can apply as much as one-and-a-half pounds of a 20-20-20 fertilizer during the course of a growing season.

Root bound plants in a landscape can become unstable as they gain height, and topple in a wind. If your hedge is root bound, plan to prune it regularly to keep it at the current size.

If you don't see the shrubs return to their old selves this year, call an arborist to look at them. They're worth the cost of an on-site consultation.

Short reports

1. Solution to the problem of odd gardening gloves.

"Forever frugal" M.M. of Westland suggests asking fellow gardeners to bring their still usable widowed right or left gloves to a gardening function. Some might be paired up with compatible mates and returned to service.

2. You know it's winter when...

...the sight of a common dandelion is reason to grin and call your friends to look-see. In the first week of January during a warm spell, J.C. of Farmington Hills found one in bloom near the house!


I enjoy answering questions you mail to me or post on my school's website. However, some problems have no solution. Don't expect much help from me if you pose a "stumper" such as: Why, in the extensive collection renowned in my family as the greatest horde of used pots anywhere, will I be unable to find the next size larger I need to up-pot a houseplant?


Green thumbs up

to opportunities such as classes, conferences, seed swap parties, lectures at local garden centers and garden volunteer planning meetings where we can gather with other gardeners during the off season. It's not only fun to see that we clean up well, but refreshing to talk in uninhibited style about growing with others who care, and appreciated by our families who need a break from our favorite topics.


Green thumbs down

to ever saying "never." To my report that "I don't know any plants that thrive on air so hot" as the exhaust from a clothes dryer, D.G. replies that her moonflowers (probably a type of Datura) seem to grow better there than anywhere else, returning each year like perennials.

P.S. Just for Barbara:

Barbara I had to gloss over the causes of needle-browning, in order to keep the Q&A within my column length. Here's another consideration:

Arborvitae shed a batch of needles every year, in autumn. Unlike spruces and junipers that lose old needles all year long, a little at a time, arbs drop the four- or five-year-old needles all at once in fall. This can be startling to people who are seeing it for the first time or just noticing it.

The oldest needles are concentrated in the center, older parts of the shrubs. As years go by if an arborvitae is quite densely branches the fallen needles may not actually fall out of the shrub and onto the ground but may clump up, log-jammed in the branches. So you may have been seeing normal needle drop, in the browning that you saw. It may simply be that so many needles have now accumulated that it's becoming hard to miss.

The jammed-up needles can even make it darker on the interior and north side of the hedge, causing more needle loss than usual.

If this is the case, you should be able to shake the needle-jams loose and watch for more browning. If it doesn't come until autumn, and all the needles that turn brown at that time are entirely within the shrub's interior or are back from the tips of the branches (that is, they are NOT the needles that just grew this year or the two previous years) then there's no problem.

If however, there is browning going on all year, and not all over but here and there, it may be the sign of stress I indicated in the published article --  branches dying from attack by twig borers or fungus that shouldn't be able to bother a really healthy arb.. But before I made that a definite, I would step back and take a look at the shrubs overall to see if their SHAPE is contributing to needle loss.

Arbs will lose needles from central and bottom areas as they become large enough to begin shading their own bases -- their "shoulders" may become wider than their "skirt." It may be that they have gotten big enough now that their newest growth at the top is geting the most sun and so it's the growth the shrubs are giving priority to. This is very likely to happen where people keep a hedge pruned with straight up-and-down sides, or sides that slant out from the base, so the top of the hedge is wider than the base. A hedge must have a taper in from a broader base to a narrower top, if it is to remain healthy It doesn't have to be a severe taper but if there is no taper at all, the lower and inner parts of the hedge will thin themselves right out to the "ugly ankles" stage while the top keeps growing strong.

I hope this helps you cover all the possibilities!

Janet Macunovich

 Originally published 1/22/05