Page 8 



BETTER FRUIT 



February 



Some Interesting Phases of the Pruning Problem 



By C. I. Lewis, Chief, Division of Horticulture, Oregon Agricultural College 



THE whole pruning question seems 

 to be enshrouded in a combination 

 of trite, perhaps platitudinal, or 

 to say the least, empirical statements; 

 such, for example, as prune in winter 

 for wood, prune in summer for fruit; 

 or heavy winter pruning stimulates or 

 encourages a greatly increased wood 

 growth and many other statements far 

 too numerous to mention. The trouble 

 seems to be that we accept many state- 

 ments like the above as fundamental 

 truths or foundation principles upon 

 which to base horticultural practices, 

 rather than recognizing that on the one 

 hand they are merely the application of 

 soem fundamental truth to a purely 

 local condition and problem, or on the 

 other hand we have failed to recognize 

 that these so-called truths are merely 

 errors or at least misinterpretations of 

 certain fundamental truths. Again, in 

 reading the standard books on pruning, 

 the more recent bulletins on the sub- 

 ject and the numerous articles written 

 by orchardists from time to time, one 

 seems at once to become lost in a maze 

 of contradictions. No sooner is an 

 opinion formulated than along comes 

 another writer reciting his experiences, 

 and lo and behold it is entirely differ- 

 ent from our previous conclusions, and 

 we begin to reorganize our conclusions. 

 A more detailed and careful study, how- 

 ever, will reveal to us that, after all, 

 these writers may all be right if we 

 understand on the one hand their local 

 conditions, or if on the other hand we 

 could accept certain statements which 

 we can prove really fundamental; in 

 other words, formulate some pruning 

 principles which we can apply under 

 all conditions. In a recent bulletin of 

 the Oregon Experiment Station, Bul- 

 letin No. 149, entitled "Vegetation and 

 Reproduction with Special Reference to 

 the Tomato," Kraus and Kraybill have 

 formulated a statement which, I believe, 

 we can accept as a code, so to speak, 

 and to which we can reconcile all our 

 experiences in pruning. This code is 

 based upon the recognition of the fact 

 that there are two classes of food which 

 enter the plant, namely, the moisture 

 and mineral food or soil nutrients, prin- 

 cipally nitrates, on the one hand, and 

 on the other the carbohydrates, the 

 sugars and starches which are syn- 

 thesized or manufactured in the leaves. 

 The interrelation or correlation, or as 

 we might say, the balance between 

 these two factors will allow us to 

 formulate certain fundamental state- 

 ments relating to general orchard prac- 

 tices, and more especially at this time, 

 pruning. There are four general con- 

 ditions of the relation of nitrates, car- 

 bohydrates and moisture within the 

 plant itself, and the responses appar- 

 ently correlated therein. These are: 



(1) "Though there be present an 

 abundance of moisture and mineral 

 nutrients, including nitrates, yet with- 

 out an available carbohydrate supply 

 vegetation is weakened and the plants 

 are non-fruitful. If, for example, either 



because of lack of storage or photo- 

 synthetic activity, the carbohydrate 

 supply were greatly reduced, even 

 though there were an abundance of 

 available moisture and nitrates, then 

 blooming and fruit production are very 

 greatly decreased, and vegetation is 

 also restricted. The suppression of 

 vegetation in itself is absolutely no 

 reason why fruitfulness should follow." 



(2) "An abundance of moisture and 

 mineral nutrients, especially nitrates, 

 coupled with an available carbohydrate 

 supply makes for increased vegetation, 

 barrenness, and sterility." 



In this case moisture, nitrates and 

 carbohydrates are all abundant. Such 

 a combination encourages rapid vege- 

 tative growth with little tendency for 

 the formation of reproductive parts. 

 "This condition differs from the pro- 

 ceeding in the availability of the car- 

 bohydrates. If, then, a pruning of any 

 type were given to trees or plants with 

 meager carbohydrate reserves or means 

 for their continued synthesis, even 

 though the nitrogen and moisture con- 

 ditions are unchanged, there would be 

 a tendency for decreased vegetation and 

 fruiting. That this is the actual situa- 

 tion is evidenced by the recorded re- 

 sults of many investigations, especially 

 those dealing with young or so-called 

 vigorously growing plants." 



(3) "A relative decrease of nitrates in 

 proportion to the carbohydrates makes 

 for an accumulation of the latter; and 

 also for fruitfulness, fertility, and 

 lessened vegetation. In such cases 

 growth is expressed both as vegetative 

 extension and specialized reproductive 

 parts, either as a sort of balance or as 

 an expression in favor of the one type 

 or the other. Compared with the pre- 

 ceding condition, actual vegetative ex- 

 tension is apparently less. It is this 

 condition which is of greatest commer- 

 cial interest to fruit growers. It is an 

 ideal condition to have trees making 

 some vegetation each year, thus in- 

 creasing and maintaining bearing area 

 coupled with abundant fruit produc- 

 tion. This nicety of balance can be 

 and is maintained through many or- 

 chard practices, especially such soil 

 treatments as will regulate nitrogen and 

 moisture conditions, and such top treat- 

 ments as can be managed through 

 pruning. Sometimes no cutting what- 

 soever may be needed, but generally 

 some is required. The desired results 

 of such cutting might be to suppress all 

 growth in one portion, encourage 

 growth in another, or to maintain a 

 balance between purely vegetative 

 parts and reproductively modified parts 

 in still others. These conditions could 

 be regulated by severe or light cutting, 

 depending upon the relative abundance 

 of the carbohydrates, nitrates and 

 moisture, pruning furnishing the most 

 ready practical means of regulating the 

 form." 



(4) "A further reduction of nitrates 

 without Inhibiting a possible increase 

 of carbohydrates makes for a sup- 



pression both of vegetation and fruit- 

 fulness. This fourth condition is most 

 frequently encountered in very old 

 trees, in those which are growing in 

 impoverished or dry soils, or in those 

 which have sustained certain types of 

 injury which virtually amount to a 

 ringing or girdling. In these cases 

 vegetative extension is notably de- 

 pressed, the foliage small and generally 

 light colored, and there is usually an 

 increased tendency toward flowering, 

 accompanied or not, as the case may be, 

 with fruit development. In many in- 

 stances there is actually a relative de- 

 crease in the production of flowers. If 

 this condition is due to a relative lack 

 of nitrates or moisture or both in pro- 

 portion to the available carbohydrates, 

 then it might be expected that if the 

 former were increased there would be 

 first a tendency toward increased vege- 

 tation and fruiting, but on increasing 

 these amounts more and more, a re- 

 sponse wholly vegetative would finally 

 result. Such increase could be brought 

 about either by some top pruning which 

 directly removes stored carbohydrates, 

 or by increasing the available nitrates 

 and moisture by the application of 

 nitrogenous fertilizers and cover crops 

 with or without irrigation, as the case 

 may be. Both these methods and their 

 gross results are well known and 

 established in practice. In general, it 

 may be added that for most tree fruits, 

 a combination of the two is most useful, 

 since if the available nitrogen is very 

 low, to secure results by pruning alone, 

 the potential bearing area must be so 

 greatly reduced that the trees are no 

 longer commercially profitable. On the 

 other hand, the application of fertilizers 

 or cover crops only, without some 

 pruning, may result in a loss of some 

 of the most profitable bearing area by 

 overcrowding or the development of 

 fruit of a poor commercial grade." 



Let us apply these four statements to 

 some phases of the pruning question. 

 We will first consider the relation of 

 the amount of pruning to certain reac- 

 tions which take place in the tree. We 

 have formed too strongly an idea, that 

 with our mature trees pruning will 

 cause a growth of unfruitful wood, or 

 at least will materially reduce the next 

 crop. This conclusion cannot be ac- 

 cepted as a fundamental truth. Its 

 acceptance, however, has led in many 

 cases to little or no pruning at all for 

 our older trees in many of our com- 

 mercial valleys, whereas such trees 

 really need much more pruning, soil 

 stimulation, fertilization and irriga- 

 tion; in fact anything which will in- 

 crease relatively the nitrates and de- 

 crease relatively the proportion of car- 

 bohydrates. Many growers also believe 

 that the number of buds, spurs, and 

 even flowers are indicative of the com- 

 ing crop and show the yielding powers 

 of the trees. Such a conclusion has led 

 and is continually leading to many 

 disappointments. We also tend to con- 

 Continued on page 26. 



