Eucalyptus as Mulch

Question from Vince:

I hope I have half as much energy when I’m your age. You asked if my UT articles are available online. I checkedsignonsandiego.com and a couple of other searches but could not find them in any organized fashion. I’ll talk to the master gardeners about posting my Plants and Pests articles on their website after it runs in the UT. This will probably take a month or so. I’ll let you know when they are available. I’ve attached the last two articles. I also checked your website and read a few of your answers to blog questions. In one inquiry about using eucalyptus tree trimmings, you said this was a good source of mulch, which it is except for one notable exception. Black walnut and English walnut and also pecan have a compound that inhibits the growth of tomatoes and a variety of other plants. Fortunately we do not have many of these trees around here. On another inquiry about ladybugs on clover you suggested planting clover as a cover crop and digging it in after the plants have bloomed. To obtain the most nitrogen from the planting, you should dig in the plants before they bloom or when they just begin. The flowering process uses nitrogen stored in the plant for seed formation. Of course if you like the flowers, it might be better to enjoy them first. Another inquiry about fertilizing in rain…there can be a problem with doing this if we have heavy rain since it can wash fertilizer both chemical and organic off the surface and into storm drains. Care should be taken where this might occur. On another inquiry about fertilizing fruit trees, you did not like blood meal and said it was too strong. I do not agree as blood meal only has 13% nitrogen and it is released as the blood meal decomposes. Fruit trees require nitrogen about one pound of actual nitrogen for most fruit trees each year. It’s hard to get this much with other organic fertilizer that have a relatively low level of nitrogen unless you apply a lot of it. Using manures can be a problem due to their salt content. Although not organic, some slow release commercial fertilizers can also be used on fruit trees. Keep up the good work.

Answer from Pat:

Thanks so much for trying to find a way to post your UT articles. And thanks also for your comments on mulches and other topics. Regarding eucalyptus as mulch, I agree that I should have mentioned that some plants do have allelopathic qualities and that of certain plants is stronger in some than others and that black walnut and English walnut have it to a great extent. The compound in walnut foliage and husks can remain active in soil for many years. I knew about black walnut since we had some of these trees on our farm back east but I had forgotten about it with English walnut, if I ever knew. I do not think, however that too many gardeners will have access to these trees. Nonetheless, I am glad you pointed this out. Twenty-five years ago when I had a section on allelopathy of trees and thus in mulch from eucalyptus in my original book that I cut that paragraph out since you pointed out to me (and rightly so) that the rotting process eventually breaks down even these complicated chemical compounds in plant matter and eventually it is all rendered harmless. So how long does it last? The compounds in eucalyptus foliage and wood break down quickly. Some people advise aging the chipped material for a week or two before using it. This may be a good precaution for a newly planted landscape. I have never seen any problem applying fresh eucalyptus clippings around established trees and shrubs. I had noticed allelopathy particularly with eucalyptus and acacia and it was in my original manuscript 25 years ago. Later after you told me that euc. was fine for mulch I used it on my paths from tree trimmers and had no problems with it. Eventually it rotted like everything else. Do you really think that I should have told that man who had spread eucalyptus under his citrus trees in Rancho Santa Fe environs that he should have removed it? No but the mulch should be kept at least a foot away from the trunk to keep the bark and crown drier. Or was your comment leveled against the following words not being totally correct? I said “Any kind of wood chips are okay to use as mulch” or words to that effect. Yes, this general statement was too broad. Maybe that is what you meant I should have mentioned that maybe not ALL since black walnut and English walnut etc. etc. Secondly, your point about digging in the cover crop before it blooms was a good one. I have used scarlet clover and waited until it bloomed simply because it’s so beautiful, but I should have mentioned the nitrogen content being better before it blooms, which is how farmers use it as green manure. Nonetheless I got great results and it rotted extremely rapidly in the ground even though it had flowered. But it is stringier then. The problem with waiting for bloom is that it makes one on a later schedule with planting summer veggies. You are absolutely correct I should have said this. I was particularly interested in your comment on fertilizer and rain. Yes, I should have mentioned storm drains, especially with clay soil. I suppose the action in my large garden on sandy soil, where rain sinks in makes me learn some things by observation that conflict with the broader facts. I had heard the advice about not fertilizing before rain and with synthetic fertilizer I can understand that, but I have always thought there was some conflict here for two reasons, one is that we always must water fertilizer into the ground right after spreading it or else much nitrogen is lost, so why not let the rains do it? You can control the amount of water applied with irrigation and prevent runoff. Rain is unpredictable. But I can see your point that with clay, it may run right off. Secondly I am thinking of things like that alfalfa and seed meals need to get into the ground and begin rotting before they can do any good so rain is helpful in that regard. Yes, organic fertilizers need to be applied earlier to allow time for decomposition so the nutrients will be available for plant use. Seed meals (cotton seed meal, etc.) don’t move much in the ground. Your points regarding fertilizing fruit trees with blood meal are of especial interest to me. I recommend blood meal for citrus and for avocado. I guess it would be okay for stone fruits as well but the amount of fertilizer one gives to a mature deciduous tree is less than for a citrus or avocado Most deciduous fruit trees should receive about one pound of actual nitrogen each year beginning with the fifth year. Some trees like figs for example do not require as much nitrogen. The amount applied can be adjusted based on the amount of shoot growth. If growth is not adequate more nitrogen can be applied the following year. Also, dwarf trees have less foliage and therefore, do not need as much nitrogen as a standard treeand as I understand it there still is not exact recommendation on how much fertilizer to give a mature deciduous fruit tree. i am saying as follows, “In warm-winter climates, feed deciduous fruit trees with a complete organic fertilizer recommended for fruit trees lightly in spring just as the flower buds swell and begin to open. What does lightly mean? You should apply enough organic fertilizer to supply the amount of nitrogen that the tree requires. The nitrogen should be available when most foliage growth occurs so you may need to apply organic fertilizers in late winter. Do not feed them in fall as recommended in cold-winter climates since this would stimulate growth instead of dormancy.” I agree. It is probably not good to feed trees with nitrogen in late fall in our area as this can delay dormancy and may stimulate new growth. If the Extension has come out with more precise instructions regarding fertilizing deciduous fruit trees I’d like to know it. I’ll check and see if anything is available. As far as citrus and avocado’s are concerned that is more clear—as much as 1 pound actual nitrogen per year for citrus and 1 and 1/2 pounds for avocado’s. (late Jan for citrus and Feb for avos, or spread out for home gardeners.)Yes, apply organic fertilizers in late winter on citrus and avocado. Applying organic nitrogen in late spring or summer on citrus can stimulate new growth which is commonly damaged by citrus leafminer during warm weather. The pest is not very active in spring and new growth should be promoted then. I also read recently in a gov’t bulletin that feeding nitrogen while citrus is actually flowering can make flowers fall off, so that aspect is what makes late Jan the safest time. But with organics maybe even earlier is better? Mid January perhaps? It depends on how fast the organic material decomposes and on weather conditions – moisture and temperature. I don’t have a good answer for all organics but would guess January is a good time to apply them. High salinity during the bloom period may interfere with fruit set. This is usually not a problem with organics except with manures which often have a high salt content. If manures are used, it is best to apply them early in December/Jan. so rain can leach out some of the salts. Quick release chemical fertilizers can damage feeder roots if too much is applied at one time and not adequately irrigated. The high salinity can also interfere with fruit set if a soluble fertilizer is applied during bloom. I am very grateful for your help and comments. Thanks for taking the time to look at my website.