PINE NEEDLES AS MULCH

[Cyril]

Hi,
We find that pine needles make a great mulch for flower or strawberry beds. Also the needles contain terpene that, we have learned from the web, has a retardant effect on germination. Thus, it is ultra helpful at discouraging weeds.
But....... We want to change our present strawberry bed to a vegetable bed. Planting out will be no problem but we are concerned about sowing seed directly into the plot.
Does the terpene eventually break down and if so how long does it take?
Any advice would be greatly appreciated.

[opabinia51]

Terpenes are organic compounds that make up what are termed "Essential Oils" of plants. Anyway, essential oils have nothing to do with any essential function of a plant but, are related to the smells associated with plants.

Anyway, the basic unit of a terpene is called 2-methyl-1,3-butadiene and sort of looks like a capital Y. Terpenes are just chains of this chemical. Based on what terpenes are in the pine needles, they can take longer periods of time to break down. For instance, if they are cyclic terpenes rather than open chain terpenes, the decomposition could take a little longer.

Chemical analysis aside; (is everyone bored yet?) just add some manure and/or grass clippings and the Ammonia (R-NH3) and Nitrates (R-NO3) will aid in the decomposition of the Terpenes in the pine needles. They should break down in a couple of months.

[grandpasrose]

Once you have done what Opa has suggested, you should probably test the ph of your soil, as pine needles can make your soil very acidic as well. You may need to add a little lime to neutralize it a bit.

Val

[opabinia51]

Hi Cyril,

I have done a little research on terpenes in pine needles and I'll give you a list of the effects of terpenes on soil:

-Monoterpenes might alter rates of nutrient cycling inhibiting nitrification in the forest soil

-In general, decomposition of secondary compounds in leaf litter is rather a slow process compared to primary metabolites (terpenes are secondary metabolites)

-Monoterpenes can be present in nearly equal concentrations in recently fallen leaf litter comapred to green needles (this quote is in conjunction with some data that eluded to the fact that resin acids and phenolic compounds retard degredation of carbon compounds more than terpenes and terpenoids)

-Monoterpenes are slightly volatile compounds and will most likely exit leaf litter via volatisation of by leaching
Furthermore, any deterrence to decomposition from monoterpenes would be short lived (say a little over a year) due to the fact that after 12 months in a Scotts Pine forest most terpenes were absent from the soil

Anyway, I have more information on monoterpenes and their effects on decomposition but, a little note on resin acids (found in pine needles):

Resin Acids have an important role in inhibiting the growth of wood rotting fungi because of their toxic effects and resin impregnation may act as nontoixc waterproofing layer that prevents fungal penetration and growth.

Anyway, these effects of Resin acids could play a role in the germination of seeds as well.

[opabinia51]

Found another article that talks about the effects of monoterpenes on the germination of plant seeds. According to the article monoterpens are toxic to vascular plants.
Furthermore, 18 volatile monoterpenes are said to be inhibitory to the germination and growth of 9 plant species. The article focuses on the inhibitory effect on the germination of corn seeds.

Anyway, here is another Journal Article on the topic:

ASPLUND, R. O. 1968. Monoterpenes: Relationship between structure and inhibition of germination. Phytochemistry 7:1995-1997.

Phytochemistry is just the name of the Journal. You will most likely have to go to your local college or university to find it.

Hope I haven't gone to over your head.

[The Helpful Gardener]

So pine poisoning IS a possibility, at least for seedlings?

Interesting Opa. I muched a bed with pine needles last year; the perennials all have returned this year, but I have NO weed seed issues at all. What 9 plants are susceptible?

Inhibiting wood fungus seems to suggest mycorhizal activity might be injured as well; any evidence there Opa? Interesting stuff indeed, and I would say Cyril's question is well answered...

Scott

[opabinia51]

Oooooh, I knew that someone would ask that question. I'll have to dig the first article up again. If I remember correctly, the researchers didn't actually say what the plants were but, they were in another article in the References. Anyway, give me a little time and I'll dig them up for you.