This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
"We have recently put new water pipes into our greenhouses, and unfortunately had them painted with coal tar, very much to the injury of our plants. Can you suggest any remedy to get rid of the trouble, or will the gas discontinue after a while. J. D. C."
The above is one of three letters received during the past thirty days from parties who have been unfortunate enough to have painted their hot water pipes with coal tar, asking what to do to get rid of it; and though I have replied by letter to each, there is but little doubt that others will fall into the same error. I thought it would be well once again to give warning through the Monthly, and at the same time state what is believed to be the only remedy when the mischief is done - namely, to take down the pipes and burn them in a heat sufficient to evaporate the tar, which penetrates deep into the iron. This can only be safely done at this season, of course, by providing some temporary means of heating, such as stoves, until the pipes can be again put up; but it is better to go to that expense and inconvenience at once than to keep firing during the whole season with the tarpainted pipes, for every cold night, when extra heat is necessary, will show the evil results in the morning by showers of dropping leaves and flowers. In your columns, again and again, has the warning against painting hot-water pipes with coal tar been given, yet every season brings its fresh victims.
From the cheapness of coal tar, and a prevailing opinion of its preservative qualities, it is often used in greenhouse construction. As far as our observation has gone, we have come to the conclusion that it is of little value, unless on outside work exposed to the air, and then only when it is used often enough to form a coating or skin on the wood work. We have used it on shelving and posts in years past, and have never been able to see that the wood ever lasted any longer than when it was not used; and as the substance is a dangerous one whenever exposed to a temperature high enough to evolve the gas, we have come to the conclusion that it is safer to keep it from the inside of the greenhouse altogether.