Question About Composting Silky Oak, Pine Needles and Juniper.

Suzi AKA DesertDance Zone 9bMarch 15, 2014

I realized I had posted this in the vermiculture forum, but we aren't raising worms, so I hope someone here can help.

We have a large Grevillea robusta Silky Oak tree that is constantly dropping leaves. I have read that the leaves have a component that makes other plants not grow. It's own seeds won't grow under it, but they pop up everywhere in our yard.

Wondering if the leaves could be composted.

We have started a pile with them, and hate to toss it all out, but maybe we just can't use them. We have also got tons of pine needles and junipers and I would like to know if they can be composted.

Thanks so much!

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All of those may be composted & will turn into compost just piled up as they are. In our zone 9 you will have finished matl. within the year. That pile can also be used to draw matl. from to make hot batches.
Compost is best when it is a mix of many things & not too much of any one thing.
Hope that helps & have fun :)

    Bookmark   March 15, 2014 at 11:09PM
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It does appear the leaves of the Silky Oak do have allelopathic, growth inhibiting, properties. Often when plant material has this composting limits it but you should do some testing on the compost you have before using it.
Possibly those leaves could be used for a mulch someplace where you do wish to suppress growth.

    Bookmark   March 16, 2014 at 6:32AM
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Suzi AKA DesertDance Zone 9b

kimmsr, how do I test the compost prior to using it?

I'm happy to know they can all be used as mulch and in the compost heap. We have plenty of all of them, and are in a drought here, so mulching is important. We were really worried about that Silk Oak and it's growth inhibiting properties.

You read so many conflicting things on the web. "NEVER use pine needles... Resin is bad stuff." "I ALWAYS use pine needles....." and on and on......

I'm taking it from the experts here, and tossing it all into the pile! We don't have a lawn, so no green from that source, but we are always pruning junipers... very green, and have lots of weeds. We live on a steep hillside that is mainly Boulders, natives and decomposing granite. We are living with what existed here when we purchased a year ago, and trying to remove and add as we can.

This is our first time with compost, so we appreciate all your answers!


    Bookmark   March 16, 2014 at 11:08AM
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By the time the composting organisms get through with the leaves, their ability to suppress other plants is very low ... those compounds are just "food" to a bacteria.

We used to have a neighbor's pine trees dropping tons of needles: they made nice compost. Same with the oleanders, junipers, and anything else of plant origin.

Pine needles make a good, durable mulch - doesn't rot quickly and doesn't mat down

    Bookmark   March 16, 2014 at 1:03PM
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interesting about pine needles.
i had never heard not to use them.
my neighbnor had a pine tree, and i took all the needles and cones i could get, and put them in my planter.
all i had were boxwoods, and i never noticed a difference.

a few years later i planted papaya in that planter.
They were pretty happy with all the pine cones that had broken way down. as far as the needles, from what i remember they took a lng time to break down, but that was a good thing as a top mulch.

    Bookmark   March 16, 2014 at 1:05PM
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Pine needles are generally avoided because they take a long time to break down thanks to their waxy coating.

Some areas/composting methods will break them down faster than others. If they work for you...they work.

    Bookmark   March 16, 2014 at 3:04PM
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Walnut leaves have some toxicity, but composted are OK. The bark and wood chips are harder to break down of course.

This is all that Wiki says about Silky Oak --
The flowers and fruit contain toxic hydrogen cyanide.[6] Tridecylresorcinol in G.robusta is responsible for contact dermatitis.[7]

KEW Royal Botanic Gardens
Grevillea robusta (silky oak)
Species information
Known hazards: The leaves are poisonous and can cause skin irritation. Cases of severe dermatitis are rare, but have been reported.

Does being poisonous equate to toxicity? Does poisonous mean poisonous to people or poisonous to plants?

    Bookmark   March 16, 2014 at 3:14PM
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seems to very much depend on the % of HCN...

I dont know much about it, but some interesting reading...



HCN applied in dosages effective ageinst insects does not affect the viability of seeds that are normally dry, with moisture contents suitable for storage. In a comprehensive study, the results of which were published between 1959 and 1961, Strong and Lindgren studied the effect of HCN in insecticidal concentrations on the germination of a wide range of seeds, including grain seeds, flax and small legumes. The variable factors considered were moisture content (8 to 14 percent), repeat fumigations and postfumigation storage. It wes concluded that germination of wheat, barley, oats, rice and flax seeds wes not impaired by one or two famigations with HCN. With the small legumes with a range of 5.8 to 12.2 percent moisture content (Ranger alfalfa, alsike clover, Ladino clover, Kenland red clever and Viking birdsfoot trefoil), all were tolerant to HCN in one or two fumigations, except aisike clover for which there was positive indication of impairment of germination.

Among 80 varieties of grain, vegetable and flower seeds tested by Lindgren et al (1955), six showed evidence of reduced germination. These were pole beans, burnet, California black mustard, smilo, marigold and snapdragon.

It may be concluded that HCN is a safe fumigant to use for seed treatment, especially for cereal grains under normal conditions, but with flower and vegetable seeds preliminary trials with local varieties are advisable.

Growing Plants and Trees

A considerable amount of injury, either temporary or permanent, may be sustained by actively growing plants fumigated with HCN. Because this gas is very soluble in water, special precautions have to be taken to reduce the amount of moisture on leaves and stems and in pots or soil balls of actively growing plants. Therefore, the plants should not be watered for one or more days before treatment. After treatment with HCN, it is necessary to wash the plants with water to remove any residual acid. HCN interferes with photosynthesis and other physiological processes (Moore and Willaman, 1917); for that reason, plants may be more susceptible to injury in daylight. Usually, treatments have to be carried out at night or in the dark. Also, following exposure, plants should be kept away from sunlight for several hours.

In the past HCN was widely used for fumigating ornamental and glasshouse plants, but it has been replaced by other fumigants that are less phytotoxic. The use of HCN generated from calcium cyanide to control glasshouse pests is discussed in Chapter 12. HCN was extensively used for may years to control scale insects on citrus trees in a tent fumigation procedure (Quayle, 1938). In this treatment, HCN gas (5 g/m ) from liquid HCN, or evolved from a salt such as calcium cyanide, was liberated into a relatively gas-tight tent installed over the tree and maintained for usually around 45 mintues. A number of factors, including temperature, humidity, physiological conditions of the tree (such as dormancy, disease, cultural conditions, stage of development and others), have considerable influence on the tolerance of the trees to the fumigant (Woglum, 1923). Since different species or varieties of citrus trees show wide variation in response and the insects in different areas may vary in tolerance, the actual conditions of treatment usually have to be developed to suit local circumstances. In scale insect eradication work, where complete kill of all insects is essential, the tent fumigation treatment with HCN has been found to be a valuable technique (Fosen et al, 1953). HCN was used by Del Rivero et al, (1974) to control larvae and eggs of the woolly white fly on orange trees.

While the concentrations of HCN required to kill insects may cause severe injury to growing plants, lower concentrations of HCN may stimulate growth and be useful in "forcing" growth in some species (Gassner, 1925).

Dormant Nursery Stock

HCN is still used in many parts of the world for the fumigation of the dormant nursery stock of deciduous trees, especially in quarantine treatments against the spread of scale insects (Ceder and Mathys, 1949; van de Pol and Rauws, 1957; Agarwala, 1956; Jen and Lai, 1959). As in growing plants, a marked stimulation has been noticed in some nursery stock fumigated with HCN (Cassner, 1925). Immediately following fumigation it is sometimes necessary to wash plants with water to avoid the burning of buds and new foliage.

Cyanide production by rhizobacteria and potential for suppression of weed seedling growth.

Here is a link that might be useful: Cyanide production by rhizobacteria and potential for suppression of weed seedling growth.

    Bookmark   March 16, 2014 at 3:44PM
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Suzi AKA DesertDance Zone 9b

Wow! That was a little deep for me, but I think those studies basically say NOT to use anything with cyanide in compost that would go near any plants or seeds you love?

But, you can use it for weed suppression?


    Bookmark   March 16, 2014 at 4:09PM
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Suzi, take some of the finished compost and put it in some small containers that will make fairly good seed starters, wet that compost, and put seeds in (beans work quite well for this). If the seeds germinate and grow the growth suppressing properties have been eliminated. If the seeds do not germinate there may still be a problem with that compost.
Do not simply assume the composting process will suppress the allelopathic properties of those leaves.
Many gardeners in North Carolina, and many other places in that area, use pine needles (called pine straw) as mulches on their gardens and are known to compost the pine needles as well.

    Bookmark   March 17, 2014 at 6:29AM
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Suzi AKA DesertDance Zone 9b

Kimmsr! Thanks for that very simple and easy test. You made my day! It will take a long time to get that compost ready to test, but it gives me hope to move forward!

If seeds start in it, we'll use it as compost. If not, it will be mulch where we want nothing to grow.



    Bookmark   March 17, 2014 at 8:51AM
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