This section is from the book "The Gardener's Monthly And Horticulturist V18", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
A few years ago I had some experience with the use of coal tar in two forcing pits, which may be interesting to some of your readers.
The wooden frame-work of the beds having become decayed, it was necessary to renew them. In order to make the new frames last as long as possible, the inside of the planks were given a coating of coal tar. The work was done in summer, so that it had time to dry and harden before the soil was put in. I had recommended to my employer pitch tar in preference to the cheaper article, fearing that bad effects might result if the high temperature in the pits should melt the coal tar. But in the fall, when the soil was placed in the beds, so hard and dry had it become that I thought the work could not be done better, or more cheaply, and my earlier apprehensions were removed. Winter-flowering plants and early vegetables occupied the beds, and all did well for about three months after being planted.
The weather having become intensely cold, the heating apparatus had to be kept at work night and day. Just at this time the plants seemed to lose their healthy look; Roses and Bouvardias began to sicken, their leaves to turn yellow and fall off. I at once suspected what the trouble was, and in order to remedy the evil as far as possible, I removed these plants out of the beds and potted them. I found that their young roots were black and lifeless from being poisoned by the tar. The continual heat from the pipes immediately under the beds had melted it, and the gases being absorbed by the soil poisoned it. In other parts of the pits, Carnations, Heliotropes and Stocks were planted in the same manner, and they did not appear to suffer in the least. Lettuces, however, didn't seem to like their quarters any better than the Roses and Bouvardias. Parsley grew quite as well as if nothing unfavorable touched its roots. I had no satisfaction from these pits that season, and when summer came round again, I took the first opportunity to remove all traces of what had given me so much trouble, and refitted the frames with hard pine planks; after which I had no trouble.
Experience is often a hard teacher, and this mishap taught me that a gardener cannot know too much about anything that relates to his business.