Growing Concerns 520: Problems that cause plant death

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A close look at the struggling arborvitaes (Thuja occidentalis variety) in this hedge may reveal mites or leaf miner at work. However, don't jump to the conclusion that insect control can save the plants. Look further, especially into light and water conditions. Arbs love steady moisture and sun. Here they are in shade part of the day (it's a winter photo so it may seem sunny, but we can see the tree trunk so we're not fooled) and they're perched on a steep sided berm. Chances are very good in this dry, low energy situation that if we use a trowel to check the root situation, we'll see there are no roots grown out of the original root ball. 

Late Spring!

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When plants die, be sure of the cause before replanting

Dear Janet,

We had 17 arborvitaes and lost all of them. They were five years old and had been healthy up to last fall, when I noticed a slight yellowing and an overall "not looking so good." I had them deep root fed at that time and in the spring, but noticed a lot more dead and dropping branches. In less than two weeks they were 90% dead.

I took a branch to the local nursery, which diagnosed spider mites. Unfortunately it was much too late to save them, and six hemlocks planted at the same time were about half gone. So I ripped everything out and replanted.

Now I wonder what to do with the new arborvitaes and sandcherries. Do I start spraying, since I am sure that mites are on the other bushes, junipers which I am treating with Isotox? Or will that harm the new bushes? Should I use a high-pressure water spray on the new ones, since I read on your website that regular water sprays can take care of mites? I really don't want to go through all of this again.


Dear K.M.,

Your frustration is understandable, yet you may be heading for another loss by leaping at the first villain you see. Take a step back to look at how several basic principles of pest management fit into your situation:

1) The presence of a pest (insect, mite or disease) does not mean it's a primary problem. Many pests are secondary to other issues; some occur or proliferate only on plants already weakened by other factors.

2) When all or many of one type of plant on a site develops a problem, review that species and variety to be sure it is right for that site.

3) When multiple species on a site are beset by a single pest, look for overall environmental problems.

If your arbs had died while other plants thrived, I'd say stop planting arbs. But hemlocks and junipers struggled, too. There may be something there that predisposes for mites, allows them to move in and multiply.

Mites like dry heat. They kill plants gradually, not as primary agents but contributors. So I'm nearly certain they are not the sole reason your plants died, not so quickly and with damage beginning and ending in cool, mite-unfriendly weather.

Mites are ever-present but often rise to problem levels on plants weakened by drought. Shady, dry conditions are a double whammy. A plant stressed by drought and low light can't make enough energy or find enough water to keep its own systems running, let alone make the extra it needs to replace what mites draw out.

Most of the years your first arbs were in place were drought years, so they may have still needed pampering when you thought they were a done deal by year two or three. Even in the best conditions plants can take years to replace roots lost in transplant and become truly self sufficient. Larger plants establish more slowly than smaller ones -- figure a year of recovery time for each inch of trunk diameter. So your plants may have been stressed for years, showing it in ways invisible to you such as lower than average growth rate until they developed severe, unmistakable symptoms at the bitter end.

Review your site to see if it offers everything an arb could want.

Is there at least six hours of sun each day? If not, prune overhanging trees or shift the shrubs to improve the light.

How does the soil drain? Arbs, hemlocks and junipers all need well drained soil. You have that if an 18 inch deep hole there, filled with water, will drain completely in less than 24 hours. They need loose, well-aerated soil, too. If the soil beyond the planting holes is hard packed, loosen it and keep it mulched.

How do you water the site? Arbs and hemlocks need steady moisture. They thrive where soil is cool and moist, never soggy, hot or dry. On slopes or hard packed soil, let water trickle so it can seep in. Be certain an area needs water by feeling the soil, don't rely on a timer or wait until plants show wilt.

Is it windy? That's stressful to arbs and hemlocks. Plant wind-loving species like sandcherry upwind, to take the edge off.

Finally, protect this big investment. Hire someone to assess the plants' health, now and a year from now.


Short report

If a plant had problems with holey leaves last year... a magnifier and look at a few of its leaf undersides now. Many pests are just beginning to feed . Deal with them now before leaves become irreversibly damaged. Also, soft solutions such as hosing with plain water and hand-squishing work better in a pest's early stages.


Green thumbs up

to on-site plant care recommendations. Much better than diagnosis from a specimen, which require the diagnostician to rely on secondhand reports of the site and history -- factors that usually have more bearing on plant trouble than a resident pest.


Green thumbs down

to hiring any plant care service without checking references. Ask for telephone numbers of satisfied customers whose contact with the service happened at least a year ago. Then ask about long term results when you call to confirm the referral.

Originally published 6/14/03

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