This section is from the book "The American Garden Vol. XI", by L. H. Bailey. Also available from Amazon: American Horticultural Society A to Z Encyclopedia of Garden Plants.
Many of our readers are actively engaged in gardening operations. To such we would say that there is no more important work than the thinning out of plants in the seed-bed, from which they are to be transplanted, or in permanent beds in the garden. This work is often seriously neglected, or too long deferred. The proper time is as soon as the plants are fairly up. Then the work can be done without injury to those that remain. When the plants first show themselves, they have no fibrous roots; consequently when they are removed, they do not disturb the soil about those that remain. But if the work is deferred until the root is a bunch of fibers, their removal breaks off many from the other plants, besides loosening the ground about the young plants remaining so that they wilt down and get a check from which they do not recover. Many a crop of beets, carrots, parsnips and turnips has been materially injured, if not ruined, in this way. "Fingered " carrots and coarse-rooted beets are, in the main, caused by deferring thinning too long. Many gardeners leave the young beets until they are large enough for "greens" before thinning. This is all wrong. Beets fit for table use can be obtained two weeks earlier if the surplus plants are pulled out as soon as they appear.
If greens are wanted, seed should be sown expressly for that purpose.
 
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