The following information on transplanting seedlings comes from Five Acres and Independence by M. G. Kains. Five Acres and Independence is also available to purchase in print.
Until about 1890 many gardening rules-of-thumb taught by the Old World apprentice system were followed blindly in America, but from that time until now these have been challenged and tested, mainly by scientists in our agricultural colleges, experiment stations and the Department of Agriculture to determine whether they are founded on scientific principles, whether they are necessary, and whether for economy’s sake they should be modified or discarded. Where only a few plants are to be transplanted the matter may be of small concern, but when thousands are to be handled, the elimination of “superfluous and frequently injurious practices” translates itself into economies that may mean the difference between profit and loss.
Among reasons that suggest the advisability of transplanting, the following seem to be reasonable, at least for American conditions. 1. Saving of costly seed and therefore money needed to plant specific areas—only an ounce or even less of some species being necessary to produce enough plants to set out an acre, whereas if sown direct in the field the quantity would run into pounds. 2. Avoidance of thinning and consequent waste of seedlings, time and money if seed were sown in the open field. 3. Prevention of the risks of open ground seeding. 4. Assurance of a good stand of transplanted plants. 5. Starting plants under glass in early spring, “pricking-out” about a month later and transplanting still later to the open field assures earliness of the crop. 6. Concentrating weed, disease and insect control and other care of the seedlings and thus reducing costs. 7. Assurance of ideal soil and other conditions in the seed bed until plants are sturdy enough for field exposure. 8. Utilization of the field by a crop previous to transplanting the one that is to follow it.
These reasons, which seem all to be well founded, indicate that transplanting is necessary, even though it may be a necessary evil! But many of the ipse dixits of Old World gardeners are so questionable that investigators have been conducting experiments to learn the truth. One of these, W. E. Loomis, in a 60-page bulletin (“Memoir 87”) of the Cornell Experiment Station, reports a two years’ investigation and series of practical and scientific experiments to determine the immediate and secondary effects.
As a preliminary to the investigation progressive growers were asked: In comparison with non-transplanting: 1. Does transplanting cause more heavy growth? Answers were, 15, yes; 11, no. 2. Does it make better root systems? Yes, 25; no, “1 (?).” 3. Does it hasten maturity? Yes, 13; no, 10. 4. Does it produce heavier yields? Yes, 5; no, 18.
These opinions, writes Loomis, show considerable divergence from earlier recommendations, when we were told to “transplant freely—nearly all vegetables are better for it”; to “transplant to induce productiveness”; and that “frequent transplanting is of great benefit but should not be done merely to give the plants more room.”
To epitomize his discussion, the experiments indicate that though strongly recommended by older gardeners and earlier writers many growers doubt the advisability of more transplantings than required in economical crop production. The tendency is away from transplanting as a cultural practice toward transplanting for economy. Early lettuce, cabbage and tomatoes are transplanted to grow them out of the normal season for a given locality. Tomatoes, sweet potatoes and similar vegetables may also be grown in higher latitudes if the plants are started with artificial heat.
Artificial protection usually requires transplanting, because it is most convenient to have the plants concentrated in a small area while the protection is being given. The same is true when special care in cultivation, watering, protection from insects and so forth are required by seedling plants. A third factor, which accounts for most of the transplanting in greenhouses and some done in intensive cultivation, is the saving of space and of expensive seeds. For these reasons transplanting will be practised as long as profitable crops can be produced by the method. On the other hand, it is an expensive operation and not to be performed unnecessarily.
The immediate effect of transplanting is to slow down or stop plant growth for a period which apparently varies directly with the amount and duration of the reduction of the water supply. When a large proportion of the active root system is retained and adequate moisture supplied, there may be little effect from transplanting. Conversely, with more susceptible plants and more rigorous treatment the check may be severe and permanent.
Recovery from transplanting is affected by environmental and internal conditions, but no factor has been found which does not appear to be based, in its final effect, on a change in the water supply of the plant. The amount of suberization [cork-ifying of plant tissues] of the older roots, the proportion of root system normally retained in transplanting, the rate of new root formation, adaptations of top to prevent water loss or increase resistance to death by sudden wilting, as well as soil moisture, temperature and other factors which may be concerned in the recovery of a plant from transplanting, all affect growth through the water supply.
After transplanting, if the plant, through abnormal accumulation of carbohydrates or other products, develops an excessive number of main root branches and a consequently reduced vigor and spread, or if it becomes stunted, either through structural changes such as lignification [conversion into woody tissues], or through permanent changes in metabolism [chemical changes in the plant], these are fundamentally the effects of a reduced water supply. Transplanting, however, is not the only operation or condition affecting water supply, so it is not surprising that there are variations in the response to this treatment.
Transpiration may apparently be significantly reduced by previous hardening treatments only when the rate of loss is low and reduction is not an important matter. Under conditions favoring rapid transpiration, water supply becomes the limiting factor, and differences between hardened and tender plants tend to disappear. The water supply concerned is the supply moved to the leaves. A reduction of the moisture content of tender leaves insufficient to cause noticeable wilting may more markedly affect the transpiration rate than all the hardening which can be given the plant.
There is, however, an important relationship between hardening and transplanting in the resistance of the hardened plants to death by drying. In those plants not capable of being hardened the intracellular moisture necessary to maintain life is less strongly held, and with the great reduction in water supply following transplanting the older leaves or, in extreme cases, the entire plant may be killed by drying.
The roots are the most important factor involved in resistance to, or recovery from, transplanting. Apparently three factors are concerned: the proportion of root system retained in transplanting—dependent upon the size of the plant and the character of the root branching; the effectiveness of the roots in absorbing water during the first few days after transplanting—an effect presumably correlated with the amount of suberization in the older roots; and the rate of new root formation—dependent upon the kind and age of plant and possibly the amount of accumulated food.
The immediate effect of transplanting is a reduction in the water supply, and the immediate and long-time results are dependent upon the severity and duration of such reduction. Therefore large plants will be more seriously injured than small ones by transplanting because the proportion of roots will be less. It follows also that those plants having root systems of such a form as to be largely lost in transplanting or whose older roots are heavily suberized so they must depend upon the outer portions for their moisture supply, will be more seriously injured than plants having a more branched and less heavily suberized root system.
No well-defined relation between the rate of water loss from the top and the ease of transplanting has been established. As an average, however, those plants easily transplanted are capable of being more thoroughly hardened. The value of such hardening probably lies more in the increased retention by the cells of the moisture necessary to maintain life than in effect upon transpiration. Hardening may also build up a food reserve to be used later in replacing the mutilated root system. Both factors are probably active in certain plants.
The rate of new root formation is the most important consideration in the reestablishment of transplanted plants. Other factors, such as absorptive capacity of the transplanted root system, resistance of the top to death by wilting, rate of growth, and susceptibility of the plant to stunting, are important only as they tend to bridge the gap between one root system and the formation of next or as their action is allowed to continue because of a slow rate of replacement.
Transplanting may be an important factor in saving valuable greenhouse or garden space, in allowing for better care of slowly developing seedlings, or in saving time or seed. While there are isolated instances of crops which appear to have made a thriftier growth after an early transplanting, these instances will seldom bear statistical analysis, so the conclusion that transplanting is a harmful but frequently necessary operation seems to be required. Conversely, the field data do not justify a large expense to avoid early transplantings, since the only shift which has consistently either earliness or total yield in cabbage and tomatoes is the final one to the field. The use of pots at this stage has produced earlier crops but has not had an important effect on total yield.
In the case of commonly transplanted crops handled when not too large and under favorable conditions, the effect of transplanting seems to be proportional to the check in the growth of the transplanted plants. When growing plants in the greenhouse this difference may be overcome by earlier sowing and is frequently lost, even in experimental work, by slight differences in watering or other conditions. It would not seem, therefore, to be important. Heavy root pruning of larger plants, transplanting under unfavorable conditions, or moving crops whose rate of root replacement is slow may seriously retard maturity and, with crops slow in root replacement, may result in permanent stunting.
To supplement the above findings and principles let me add a few suggestions of a practical nature, such as deal with the actual operation.* Every experienced man knows them, so the sooner you as a beginner learn them the better; for you should keep them well in mind whenever you have work of this kind to do. [*These are mostly quoted from the author’s Modern Guide to Successful Gardening.]
Newly turned, deeply worked soil favors transplanting because the roots quickly become established in it. Good plants—sturdy, stocky ones—are surer to succeed than spindly ones, which latter are likely to collapse. When the soil is dry, cool, cloudy days favor success, so do late afternoon and early evening. When possible, transplant just before a rain. Never transplant just after because the soil, unless sandy, will puddle and bake. Wait until the surface has lost its excess moisture. Always press the earth firmly around the roots but leave the immediate surface loose to check evaporation.
Certain species of plants are difficult to transplant successfully (melon, squash)—those which the seed packet directions say: “Sow where the plants are to remain.” However, some of these may be successfully transplanted if their seeds are sown in 6″ squares of inverted sod, in strawberry baskets or paper pots 4 to 6 weeks before time to set in the garden. When transplanting do not disturb the roots. Paper pots are not removed because they soon decay; but strawberry boxes must be cut at their corners and the pieces carefully slid from below. After planting water such plants until they become established.
At least 3 hours (preferably longer) before you plan to remove seedlings or transplant potted plants soak the soil thoroughly so the plants will be filled with water and so earth will cling to their roots. Whenever possible do transplanting outdoors on a cloudy day, before a rain, toward evening, or after sundown and, unless the soil is moist, water the plants as soon as set.
After the seedlings in the flats have developed 2 to 4 leaves prick them out 1″ to 2″ apart each way with their roots extended downward full length, never curled up, in another flat and keep them fairly cool so they may develop stockily and sturdily.
For very small seedlings a large crochet needle or a double pointed stick will lift and place the plants more speedily than fingers. Seedlings 1″ to 2″ high are best pricked out with a dibble about as thick as one’s middle finger but more pointed. When the plants are 4″ to 6″ high a flat trowel is better than a dibble because when skillfully used there is less danger of leaving an airspace around the roots. Always press the soil from the bottom upward and by a sidewise push to make it firm around all the roots. Avoid surface thrusts downward as these are likely to make air pockets.
When the leaves again touch prick out each alternate plant into other flats or outdoors if conditions are favorable but place the flats in a coldframe otherwise.
The day before transplanting to the open ground thoroughly soak the soil so the plants will be full of water. To remove them first tilt the flat and lift it a few inches, then strike its bottom edge of one side on the ground hard enough to shift the soil and plants a little so as to break connection of the soil with the sides and bottom. Then lay it flat on the ground and remove a few plants at one corner. After these are out the others may be easily lifted with goodly lumps of soil attached to their roots.
Puddling (dipping the roots in thin mud) is usually not a good practice with small plants because it bunches the roots instead of keeping them separate. When plants are lifted with little or no earth attached to them their tops should be watered and they should be protected from sun and wind with wet burlap. Clipping about half the tops is a good practice with large leaved plants, especially those lifted without much earth attached. It is also a good plan to set spindly plants as deep as their seed leaves, particularly when the soil is rather dry. This places the roots in moister, cooler soil than does shallow setting and tends to keep them from falling over.
In a small way and in dry soil plants may be shaded from the sun for a day or so after being transplanted by thrusting a shingle slantingly into the ground on the south side of each. But when the soil is moist and other conditions favorable this is not necessary. What is more important is to press the earth firmly around the roots and to leave the surface earth loose to serve as a mulch.
For large scale transplanting various power machines are available. They do excellent work. On a smaller scale I have found a metal plant setter more satisfactory than either trowel or dibble in stony land. It drops, sets and waters each plant as the operator strolls slowly across the field. In well prepared stone-free soil from 5,000 to 8,000 plants may be set in a day.