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Fertilizing and Watering

Subject:  Coffee Grounds

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Date Posted


Eau Claire, WI

First post,
First year,
First attempt!

I've been reading like a madman. I don't have a lot of space, but I am doing what I can. As with other plants I dug in a good amount of partially composted kitchen waste for a sustained feeding. I fortified with mycorrhizal fungi, and I could go on, but the long and short of the question is about coffee grounds. I've always included them in my compost - say 10 - 20% I just spent the last two hours reading about how coffee grounds are both great AND terrible for plant growth. Folklore likes them, but the science seems to bend toward the bad end of the scale.

Here is a tiny recap of my research:
Acidify the soil - somewhat, but I can balance with gypsum.
Stave off pests like slugs, that's nice
Slows growth and can inhibit roots.
Worms love them - Great if I can keep the moles out.
Inhibit fungal disease, ok, but then what about mycorrhizals?
Inhibit Weeds? whats food for the goose...
Nitrogen - Yea
Caffeine - maybe not so good.

I'm curious what BigPumpkins wisdom (opinions) will yield.

- Pepo

5/24/2018 11:45:30 PM


Syracuse, NY

Until someone with info from this decade chimes in, you can go to the home page search window, type in coffee grounds and then hit enter; on THAT page, go allllll the way to the bottom (you may be able to hit 'Page Down' 5/8 of the way across your keyboard on the right to go MUCH faster) and then go upward from there because at the TOP are mostly pictures of everybody's coffee grounds and you don't want that. i also briefly read on Google that USED coffee grounds are nearly neutral or about 6.8 pH and UNUSED are acidic very much. i just looked at your typing again and was like WOW---you ARE a madman! Good luck getting answers and welcome to BigPumpkins.com! eg

5/25/2018 1:01:53 AM

Glenoma Kins

Southwest WA

Aged compost works better than raw additions generally, and I think coffee grounds are no exception. Spreading them on the soil surface would be your best bet imho also keep in mind pumpkins need only moderate levels of nitrogen. Welcome to bp and pumpkin growing!

5/25/2018 2:33:56 AM


Eau Claire, WI

Thanks I appreciate the replies.

Glenomkins, from further reading, I agree that composting spent grounds is best. The longer the better. 20% max.

pumpkinpal2, thanks for the pointer, I don’t know how I missed all the coffee grounds posts. I did go back and read most of them. They are almost all positive and mostly say the same thing, ‘coffee grounds are good, and worms love them.’ Lots of folks burry them directly, or add them as mulch. I don’t think either is a good idea. I agree that PH is not much of a concern.

I did want to balance the extremely positive feeling here about coffee grounds with more current wisdom and science from this decade, so I’ll take your invite to chime in! 😊

The top snippets are from gardening or news sites. The last few are from university or soil test papers. Much of this is biased against the heavy use of coffee grounds, since that’s was I was looking for and it’s more rare than folk wisdom of more=better. Anyway I wanted to share what I learned and include references. All posts telling me I’m crazy are welcome – it’s nothing new.

I'm way over the character count so I'll split this into a few posts.

- Pepo

5/25/2018 11:13:51 PM


Eau Claire, WI

Most organic gardeners agree that adding coffee ground to your plants is a "good thing. There is little published information as to how the ground should be applied. Coffee grounds are organic and will break down just as compost does. When a living thing dies, it gives back the nutrients is has stored during its lifetime. Coffee grounds contain 2% nitrogen and less than 1% each of phosphorus and potassium.

Researchers have discovered that these little bits of organic material can indeed affect plants. In fact coffee ground can affect different plants in very different ways. For example lettuce seems to benefit from coffee grounds. It seemed to have little effect on alfalfa. Tomato plants were negatively affected by application of grounds. This effect seemed to be due to an ingredient in coffee grounds that acts as an allelopathic chemical, which inhibits the growth of some plants. Fresh coffee grounds can tie up nitrogen that the plant might otherwise take up.

Sprinkling coffee grounds on the soil surface probably won't hurt anything; but expecting it to change the soil pH without mixing it into the soil probably won't happen. However if you do mix it into the soil could inhibit root growth due to the allelopathhic chemical factor depending on the plant. Do you want to take that chance?

5/25/2018 11:14:36 PM


Eau Claire, WI

And the results? Well, here’s the deal. The crop yield and growth of pretty much everything in the coffee bed became noticeably worse within about two weeks of application. Plant growth slowed, some developed leaf yellowing, others defoliated and died. Seedling germination in some cases was almost completely inhibited. While some species looked OK, none of the plants in the coffee group proved better than my basic control. But it’s just adding organic matter. What went wrong?

So I had a look at the scientific literature, and frankly I kicked myself. Coffee grounds are of course a rich source of caffeine – in fact they can be richer than coffee itself, depending on brewing technique. One of the key functions of caffeine in the plants that produce it is allelopathy – the ability to reduce competition from surrounding species by suppressing their growth. Caffeine is packed into coffee seeds for the very function of suppressing the germination of other seeds.

There is a stack of studies to suggest it also stalls root growth in young plants, preventing their uptake of water and nutrients. Yet others have shown it has antibacterial effects (so much for boosting soil bacteria). And guess what? It isn’t even always very acidic. OK, its effects have varied widely depending on plant species, but it’s never shown colossal benefits that could outweigh the risks. I love a quirky piece of hort advice, and some are repeated so often you assume they are true, but often they call them old wives’ tales for a reason.

5/25/2018 11:15:34 PM


Eau Claire, WI

Myth: Coffee grounds in the soil will help my plants grow better.

Realty: Coffee grounds may benefit some plants (as they break down they add some great organic byproducts), but in many cases they don’t help and might harm the plant. Use coffee grounds sparingly around plants or in your compost pile if wanting to recycle them. Mix them with another organic product if using it as a topical mulch.
– Amy Jo Detweiler, OSU Extension horticulturist

5/25/2018 11:16:44 PM


Eau Claire, WI

"Less straightforward are the changes in pH that occur during decomposition. A commonly held assumption states that coffee grounds are acidic, but this does not hold true experimentally. While two studies on coffee ground composting reported mildly acidic pHs of 4.6 and 5.26, others have measured neutral (7.7) to somewhat alkaline (8.4) pH levels. One researcher found that the pH of soil treated with coffee compost increased after 14 to 21 days of incubation, gradually decreasing thereafter. Obviously the pH of decomposing coffee grounds is not stable and one shouldn’t assume that it will always, or ever, be acidic.”

They will compost fairly quickly as someone else mentioned due to their high surface area to volume ratio. While you would always want to avoid placing compostable material near the main stem or trunk of plants and trees, this is especially the case with something that can decompose quickly and cause temporary spikes in the organism balance there -- which may sometimes include large numbers of things that feed on live plants as a secondary target, or otherwise cause short-term negative effects.

Prior to composting at least moderately, they inhibit seed germination and early plant development a lot. So if you apply them directly only do so to trees not vegetable garden. If you apply to vegetable garden make sure you get it into the soil and wait a month before placing seeds. You can use this to your advantage though by reducing weed proliferation later in a season or around your trees.

Just be careful because the coffee ground does contain an allelochemical. It is a naturally produced chemical that the coffee plant uses to eliminate competition. It does not affect every plants but if you use coffee ground with sensitive plants you might kill them or prevent germination of their seeds.
The allelochemical could be broken down by

5/25/2018 11:17:55 PM


Eau Claire, WI

Disease suppression
As they decompose, coffee grounds appear to suppress some common fungal rots and wilts, including Fusarium , Pythium , and Sclerotinia species. In these studies, coffee grounds were part of a compost mix, in one case comprising as little as 0.5 percent of the material. Researchers suggest that the bacterial and fungal species normally found on decomposing coffee grounds, such as non-pathogenic Pseudomonas, Fusarium, and Trichoderma spp. and pin molds (Mucorales), prevent pathogenic fungi from establishing. A similar biocontrol effect was noted on bacterial pathogens including E. coli and Staphylococcus spp., which were reduced on ripening cheeses covered with coffee grounds. Currently, disease suppression from coffee grounds has only been demonstrated under controlled conditions on a handful of vegetable crops, including bean, cucumber, spinach, and tomato. Their efficacy in gardens and landscapes is unknown, as is any protective activity on other plant materials such as trees or shrubs.

5/25/2018 11:18:34 PM


Eau Claire, WI


Not all get a jolt
Not all plants get a jolt from coffee grounds. Seed germination of alfalfa (Medicago sativa) and white and red clovers (Trifolium repens and T. pratense) was inhibited by water leached through coffee grounds. Growth of crops such as Chinese mustard (Brassica juncea), komatsuna (Brassica campestris) and Italian ryegrass (Lolium multiflorum) were all inhibited by coffee grounds, as was that of ornamentals including inch plant (Tradescantia albiflora), geranium, and asparagus fern. One investigator speculated that toxic substances released from decomposing coffee grounds were responsible for their inhibitory effect. This effect also reduces weeds, and perhaps in a landscape dominated by large shrubs and trees, only germinating seeds and seedlings would be injured. But as there has been no experimental research on coffee grounds and woody plants, this is only speculation on my part

In compost
• Percentages of 10 to 20 percent of total compost volume have been reported as optimal for compost quality and effectiveness, while over 30 percent can be detrimental.
• Only small amounts of coffee grounds are required for effective disease suppression. Therefore, I recommend using no more than 20% by volume of coffee grounds in a compost pile. A diverse feedstock will ensure a diversity of microorganisms.
• Don’t assume coffee grounds will make an acidic compost; pH levels will undoubtedly change over time.

For mulching
• Coffee grounds are finely textured and easily compacted.
• Compacted coffee grounds create a barrier to moisture and air movement, especially when applied in thick layers.
• Therefore, I recommend against using pure coffee grounds as a mulch; instead, try using a thin layer (no more than half an inch) of coffee grounds and cover with a thicker (four inches) layer of coarse organic mulch like wood chips.

5/25/2018 11:19:47 PM


Eau Claire, WI

However, new Australian research shows that spent coffee grounds, applied fresh to garden beds, decreases the growth and development of seedlings.

Sarah Hardgrove, at University of Melbourne’s Burnley campus, says: “I wanted to investigate the potential for spent coffee grounds to provide similar plant growth and soil property benefits as other organic amendments such as manures, biochar, worm castings and compost. The trials were designed to investigate the impacts on different plants with varying nutrient requirements and pH preferences.”

Hardgrove trialled broccoli, radish, leek, viola and sunflower seedlings in greenhouse pots and in the field, testing fertiliser alone, fertiliser plus coffee grounds, and coffee grounds at 2.5, 5, 10 and 20 per cent by volume, measured against the control with no additives. Significantly, all the plants grown in the coffee-amended soil treatments showed poor growth compared with those without coffee grounds. On the positive side, coffee grounds increased soil water-holding capacity.

Why was plant growth inhibited? Further tests showed the soil pH of the soil was not a factor. Nitrogen drawdown, where decomposer micro-organisms use up the available nitrogen in the soil, depleting the supply for plants, could not account for it either. Hardgrove suggests the likeliest explanation is phytotoxic¬ity, where naturally produced plant toxins injure other plants. Phytotoxicity is relatively common: some well-known examples include black walnut trees, casuarinas, conifers and bottlebrushes that inhibit the growth of other plants beneath their canopies.

Hardgrove’s conclusion is that adding more than about 2 per cent by volume of spent coffee grounds to soil will likely decrease plant growth in the short term.

5/25/2018 11:22:20 PM


Eau Claire, WI

...Continued from above

Note that these trials were done with seedlings and the effects on mature plants have not yet been tested.

Intriguingly, a Japanese study last year indicated that after an initial period of stunting plants, coffee grounds began to show beneficial effects, doubling crop growth after six months.

http://www.sgaonline.org.au/using-coffee-grounds-in-the-garden/ WITH PICTURES:

The experiment
In a greenhouse pot trial, broccoli, radish, leek, viola and sunflower (chosen for their varied nutrient-pH preferences) were grown in sand, loam and sandy clay loam substrates. Four treatments were applied: no treatment control, spent coffee grounds (5% volume), fertiliser and spent coffee grounds plus fertiliser.

Concurrently, a field trial grew the same plants under six treatments: control, fertiliser, and spent coffee grounds at 2.5%, 5%, 10% and 20% volume application rates (in the upper 10cm of soil).

Plant Growth
In the greenhouse trial, all plants grown in coffee-amended soil treatments showed poor growth compared to the control and fertiliser-amended soil treatments. The left hand picture below shows the five plants under the four treatments, from top to bottom: control, fertiliser; spent coffee grounds; spent coffee grounds plus fertiliser.

In the field trial similar results were obtained. The right hand picture below shows, from left to right, Control, Fertiliser, Coffee @ 2.5%; @5%; @10% and @ 20%.

There you have it. It's not definitive , but its worth considering.

- Pepo

5/25/2018 11:23:20 PM


Syracuse, NY

omG. Omg. OMG!
"so I’ll take your invite to chime in! 😊"
ummm, yeah, my comment was based upon the fact that when i used the search window on the home page, all of the results i saw were from back in '03, '06, etc., so that process i provided was to give you something to chew on until any other growers had their comments to tell you in the next day or so.
I picked a bad year to quit drinking coffee, lol! I WILL have a beer, though, after having seen this page at 4 am as i watch a move that i was supposed to have started 3 hours ago...but, you know how BigPumpkins.com is an all-nighter-----i and others will certainly benefit from your research! Now, between quitting coffee and drinking less...what WILL i have left? oh, yeah, pumpkins...ha ha ha eric g

5/26/2018 4:54:07 AM


Central Illinois

grow a container plant that has a lot of coffee grounds in it and see what effects you get

5/26/2018 8:56:49 AM


Eau Claire, WI

Baitman, yeah, the only real test is a side by side comparison. I'm guessing a lot of folks here will discount all of this because they have been using coffee grounds for years and growing great pumpkins, but without comparisons it's impossible to judge. A community test would be nice. I could do one on sprouting seeds and young growth, but I don't have the space to see it to completion. There are studies that show increased growth after initial stunting, so a complete season trial would be a good test as well.

An interesting positive from the last snippet was:

"Intriguingly, a Japanese study last year indicated that after an initial period of stunting plants,
coffee grounds began to show beneficial effects, doubling crop growth after six months."

One big takeaway from all of this is that these effects seem to be plant specific. So what are the specific pumpkin effects? Inquiring minds want to know!


5/26/2018 11:59:15 AM

Glenoma Kins

Southwest WA

Tomatoes negatively affected... thanks good tip there. As a worm some coffee would definitely be the highlight of ones life...? Maybe being a worm wouldn’t be so bad after all :)

5/26/2018 1:56:36 PM

Glenoma Kins

Southwest WA

Wow. Basically don’t direct-apply coffee grounds except to suppress weeds. I think that answers another post about how to suppress weeds. I did use them one time on a pumpkin and they did not help. Thanks for the good info Pepo.

5/26/2018 2:16:59 PM

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