That greatest of garden smells

Last updated 12:22 13/07/2012
coffee plants
GROW YOUR OWN: Coffee plants produce an abundance of fragrant white blooms but need frost-free conditions.

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A coffee lover from way back, I find some brews smell better than they taste. Sometimes it's the beans and sometimes it's the brew, but whatever the taste of the coffee, I always love the smell.

And the magic of the smell goes way back to days working at Dunedin Botanic Gardens. They were fun, kind of crazy days, working with some top horticultural types and a random mix of student dropouts filling in time. And among the tasks of choice, the most popular was always working in the rose gardens. That's because they were regularly mulched with spent coffee beans from the local Gregg's coffee company.

Freshly roasted, still-warm beans would arrive by the truckload to be delivered for spreading around roses. It was a blissful task, especially on cold winter mornings when we would do a few bean dives, throwing ourselves into the pile of warm, aromatic coffee and inhale.

As garden smells go, it has to be the best, and it probably spoilt me for any future contender for the title. And when they were in flower, the roses smelt quite good too.

The coffee bean mulch had the added appeal of its dark brown colour and light, friable texture, ideal for aeration, and it was excellent at suppressing weed growth and maintaining summer soil moisture. It gradually broke down to boost the soil organic matter, which is just what you want in good mulch.

While it won't give you an absolutely maintenance-free garden, mulch will certainly ease the burden of weeding and cost of watering. And maintaining soil moisture is good for your plants, keeping their surface roots free from moisture stress.

It also makes a better home for the worms and fellow microorganisms.

Just about anything organic will do as a mulch, but not all are what they might seem. It pays to know just exactly what you're getting.

Years ago, a daffodil grower near Nelson called about problems with his bulbs.

The newly opened blooms were distorted and split, as if affected by hormone herbicide. But it was a mystery how it could have happened. The experienced grower didn't dare use herbicides near his prized plants and had kept detailed records from years of growing and breeding bulbs. He carefully hand weeded the beds and fed the plants with special, well-proven fertiliser.

Looking around the area, there was no sign of where spray might have drifted from, if that was the cause.

And, despite extra care, the following year the symptoms were worse. Again the leaves and blooms came up distorted, split and ruined. Yet no diseases were apparent and again there was no sign of herbicide drift.

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It was a mystery, until a fellow daffodil grower who had previously grown peas hit upon a possible cause; pea straw. Covering the beds of daffodils was a mulch of pea straw, replenished each year with a fresh layer.

And to grow peas at the time, a typical grower used pre and post-emergent selective herbicide, a common practice, even after germination. While the use of the herbicide was legitimate and the peas themselves may have been unaffected as they grew, the symptoms appearing on the daffodils, although not fatal, were demonstrably typical of herbicide damage.

The implication was that the herbicide remained persistent in the pea straw, either within the cellular structure or on the surface of the dead organic matter, and, as the straw broke down, was released into the soil of the daffodil beds.

Applying the pea straw to the soil as mulch each year, the daffodils were potentially being dosed with more and more of the causative agent. And, because daffodils are storage organs, absorbing nutrients and whatever else from their roots and leaves as they die back each year, the dose, if it was from the pea straw, simply compounded.

However, nothing was tested or proven as coming from a herbicide.

It could be argued that with daffodils – even prize winning ones – the results are largely temporary, and, once the causative agent was withdrawn, they could in time be expected to out grow the effects and eventually bounce back. But even if there was no permanent damage to their DNA or their show and breeding potential, the implications around use of pea straw mulch on them remained.

And, if that same pea straw had been used on a vegetable patch, in particular other storage plants such as onions and garlic, what would the effects be?

The one redeeming factor is that with vegetables, the crop is grown once, then harvested, not re-grown year after year like daffodils, so the effects are not cumulative in the individual plants. However, if tested, a soil might tell a different story.

Looking up the agrichemicals that can be used to grow peas and onions today can be a little alarming, but such is the way of the world where consumer demands at acceptable prices necessitate certain cultural practices.

Or you can go the organic way and probably pay more up front for your peas and the straw.

But probably the most important consideration is to inform yourself about the things you put on your soil and, potentially, on your edible crops.

Coffee beans may be a favourite, but they are not readily available. Pea straw makes excellent mulch, but if using it, you may want to be satisfied it will do an appropriate and safe job for you.

Call it a cliche, but even in the garden there's no free lunch.


Try growing your own coffee beans. The tropical evergreen will grow outside in a frost-free situation or indoors in a pot but needs plenty of room to grow and regular liquid fertiliser to flower well and, hopefully, produce fruit and beans. If not, the clusters of beautiful white flowers have a delicious, sweet perfume. Use coffee beans, if you can get them, as a mulch, but because they are have a slightly acidic pH, add a regular dressing of lime to avoid over-acidic soils if you use it repeatedly.

If you have a compost heap, give it the once-over this month, turning it to mix and aerate the contents and help speed up decomposition ready for spreading next month prior to the spring planting season. If it's a bit slow, give it a sprinkling of readily available nitrogen fertiliser such as dried blood or sheepy-do to hurry things along.

If you've grown a winter cover crop in the vege garden it will need to be dug in at least a month before you plan to start planting, so make sure it's done this month if you want to plant your garden in August. Buds on dormant trees and shrubs will start to move at the end of this month so time is running out for winter pruning. Make sure you're on track to have it all done before August.

- Nelson

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