The Sceptical Gardener

The Thinking Person's Guide to Good Gardening

Ken Thompson

• Publisher: Icon Books
• Year: 2015
• ISBN:
  9781848319349
• Topic: Biological Sciences
• Subtopic: Horticulture
• Index: Biological Sciences

How are birds linked to house prices?
How can a gardener improve the
flavour of their vegetables?
Do wildflowers really thrive in poor soil? In this collection of articles from The Telegraph, biologist and gardening columnist Ken Thompson takes a scientific look at some of the greater – and lesser – questions faced by gardeners everywhere in a bid to sort the genuine wisdom from the hokum.What is the ideal temperature for a compost heap? What do bees do that improves strawberries? Why are gardeners in literature always such dummies? This is an expert's gardening miscellany, aimed at making you not necessarily a better gardener, but probably a far more thoughtful one.

PRACTICAL GARDENING

Compost – getting the temperature right

Making your own compost turns waste into a valuable material that supplies nutrients, improves soil structure and suppresses weeds. So most of us do it and, if they’re being honest, even those who don’t do it know they should.

But there has always been a good degree of (justifiable) paranoia about what can safely go on the compost heap. Essentially it’s a question of temperature – a hot compost heap kills most things, but a cool one doesn’t. Sometimes there are ways round this. Roots and rhizomes of perennial weeds will survive in a cool compost heap, but you can always kill them first. One way to do this, and to my mind the most satisfying, is to beat them to a pulp with a hammer, but just leaving them somewhere dry until thoroughly shrivelled works just as well in the end. But there is no such easy solution for weed seeds, and those that survive for long periods in soil (which is most of them, sadly) will do exactly the same in a cool compost heap.

Plant pests and diseases have always been problematical too, especially with the arrival of a whole raft of new killers that we need to worry about – sudden oak death (Phytophthora ramorum) and ash dieback (Chalara fraxinea), for example. Again, temperature is the key, so how hot does a heap need to be? Research shows that pests and diseases vary in their temperature tolerance, but that 50°C for at least seven days is enough to see off Phytophthora (including potato blight), clubroot, nematodes, the various organisms that cause damping-off, plus larvae of carrot, onion and narcissus flies. But you need a really hot compost heap to kill viruses, perhaps because they’re hardly alive in the first place.

So much for the good news. How likely is your compost heap to attain 50°C for seven days? Not very likely is the answer. The crucial difficulty, for the average gardener, is size. Paul Alexander, the Royal Horticultural Society’s compost wizard, compared open heaps, small plastic bins (like the ones often supplied by local authorities) and traditional wooden bins, all filled with a 50/50 mixture of shredded woody and soft green waste collected from the RHS garden at Wisley. There were subtle differences between bin types, with the wooden bin being slightly warmer, although it’s hard to say whether this was because it was rather larger or because wood is a better insulator. Turning helped too, with bins turned once a month being slightly warmer than those left unturned. But the bottom line is that none of the bins got more than a few degrees above air temperature, and certainly none got anywhere near 50°C. The raw material was clearly fine, because a giant 70 cubic metre heap of the same stuff stayed above 40°C, and most of the time above 50°C, for a whole year.

The small compost heaps, typical of the size found in the average garden, stayed cold because they have too much surface area relative to their volume. In order to heat up reliably, a compost heap almost certainly needs to be significantly bigger than 1 cubic metre. It also needs to be filled up pretty quickly, since a heap filled slowly over a few weeks (the normal garden pattern) stands even less chance of becoming hot. A final problem to bear in mind is that even a hot compost heap will be much cooler at the surface, the edges and especially (in a square bin) the corners, so the heap needs to be turned carefully so that these outer parts end up in the middle. All this makes proper ‘hot’ composting hard work in the average-sized garden.

What is the gardener to do? All the owner of a normal, cool compost heap can hope to do is try to stop diseases and weed seeds getting into the heap in the first place. The determined compost cooker needs a bin that is specially designed to keep the heat in. The recently introduced HOTBIN® is designed to do just that, essentially by being heavily insulated. I haven’t tried a HOTBIN myself, but everything I hear (from Which?, for example) suggests they work a treat.

Conifers and soil pH

I’m sometimes asked if conifers can acidify the soil underneath them. The short answer is yes, but as usual the long answer is not quite so simple. Note first of all that it’s a waste of time just to wander out into the countryside and start measuring soil pH under different trees, because trees naturally sort themselves out on to the soils that suit them, so cause and effect cannot be inferred from simple observations.

What is required is an experimental approach. And also, because everyone agrees that if trees modify soil pH, it doesn’t happen overnight, plenty of patience – which in practice means finding some trees that someone else planted sometime in the past. The best study that I know of was carried out in Poland by a joint Polish–American team and published in 2005 in the journal Ecology Letters. Replicate blocks of fourteen tree species (eight broadleaves and six conifers) had been planted in 1970, and the researchers went back 30 years later to see what had happened to the soil. Before the land was cleared for the experiment it was a Scots pine forest with an acid, sandy loam soil.

The effects of the different trees on the soil were dramatic. Underneath the conifers, the soil remained acid, and in some cases had become even more acid. The most acid soils were beneath Scots pine, Douglas fir, larch and Austrian pine (Pinus nigra). Underneath the broadleaves, the soil generally became more alkaline, sometimes markedly so. The key variable turned out to be the amount of calcium in the leaves of the tree – the more calcium, the higher the soil pH. The tree with the most calcium-rich foliage, and which also produced the biggest rise in soil pH, was small-leaved lime, Tilia cordata, but sycamore and Norway maple weren’t far behind.

The only important variable was foliage calcium, and once you know that, tree type itself (broadleaf or conifer, evergreen or deciduous) wasn’t important. Thus although conifers generally lowered soil pH, and broadleaves raised it, the two groups overlapped. The conifer with the most calcium-rich leaves (silver fir, Abies alba) had about the same calcium as both pedunculate and red oak, and had a similar effect on soil pH. Hornbeam had even less calcium in its leaves, and produced the lowest soil pH of all the broad-leaves. So the answer to the question ‘do conifers make the soil below them more acid?’ is generally yes, but there’s a lot of variation between species, and not all conifers (or broadleaves) behave the same.

Moreover, there was evidence of positive feedback between trees and soil. They measured leaf calcium in 1995 and again in 2001, and found that trees with low-calcium leaves stayed the same, but the calcium content of calcium-rich leaves actually increased. So calcium in leaves raises soil pH, which further increases calcium in leaves, which raises soil pH … and so on. Finally, all this had a predictable effect on earthworms: in the soil beneath the high-calcium broadleaves, there were lots, of several species. Beneath pine and larch, there were essentially none.

There are lessons here for gardeners, even if they’re not quite obvious. Don’t imagine you can render a chalky soil rhododendron-friendly by planting a few pine trees – it simply takes too long. Remember too that an acid soil is a fragile thing, convertible overnight into an alkaline one by the addition of a few buckets of lime, so even the soil under an old pine tree may or may not be acid, depending on what else you, or a previous gardener, have been adding to the soil.

There’s a lesson for composters too. For reasons that remain mysterious (to me anyway), the compost made from general garden waste often turns out to be surprisingly alkaline. But even if it takes ages to compost conifer needles, I think we can safely predict that the finished product will be acid.

Crocks in pots

‘[F]ill the bottom of the pot with extra drainage material, such as polystyrene pieces or crocks …’, says the RHS book How to Garden. Most people seem to agree: the BBC Gardeners’ World website has instructions on planting up scores of different kinds of pots, but all begin with some variation on ‘Place a layer of crocks in the bottom of the pot to improve drainage’.

So there you are, summer is just around the corner and you’re happily chucking broken crocks into the bottom of a plant container before adding some compost. While you’re doing this, ‘textural discontinuities’, ‘capillary barriers’ and ‘funnelled flow’ are probably not uppermost in your mind. But maybe they should be. Soil scientists, hydrologists and environmental engineers have long known that peculiar things happen at the junction between two layers of soil with different textures, and especially when a fine layer sits on top of a coarse layer. For example, scientists trying to track the movement of fertilisers, pesticides or other contaminants down soil profiles sometimes find that if the stuff they’re following encounters such a discontinuity (especially if it’s not perfectly level), it can stop heading downwards and zip off sideways, ending up a long way from where they expected to find it.

Fair enough, you may think, but what has that got to with me, and can I go back to planting up my pots? Yes, in a minute, but first here’s another funny thing. Because it resists compaction and provides good drainage, sand is the basis of most modern golf course putting greens. But the downside of sand is that it holds little water, dries out rapidly and needs a lot of watering. The most popular solution to this problem is around 300 mm of sand over a 100 mm layer of gravel. Capillary forces within the sand mean that water is unwilling to cross from the (relatively fine) sand to the (much coarser) gravel, creating what hydrologists and geologists call a ‘perched’ water table, essentially one that is higher up than it should be, and above the ‘real’ water table.

Maybe you’re now starting to see the parallel between the sand and gravel beneath a putting green and the compost and crocks in your plant pot. Both are a fine layer over a coarse layer. But the former is designed to reduce water loss from the fine layer and keep it wetter than it would otherwise be, while the latter, if we believe the gardening books, is to improve drainage and keep the fine layer drier. They can’t both be right, although in a sense they are. During heavy rain, the putting-green sand layer eventually becomes saturated, gravity overcomes capillary forces and the water has nowhere else to go but into the gravel, where it drains away rapidly. So the sand/gravel sandwich is well-drained. But once the surplus water has drained away, the sand remains wetter than it would be if it were just sitting on more sand.

Exactly the same happens in your plant pot. When you pour enough water in the top of the pot to saturate the compost, gravity overcomes the capillary barrier at the compost/crocks boundary and it drains away through the crocks and out of the drainage hole. But it would do exactly the same if the crocks weren’t there, and when you stop watering, you’re left with a perched water table in either case, crocks or no crocks. The only difference is that if there’s a layer of crocks, the water table is perched at the compost/crocks boundary, and if there isn’t, it’s at the bottom of the pot. So there’s no harm in continuing to bung crocks in the bottom of containers if you feel you ought to, or because your mother did, and her mother before her, but be aware that their only practical effect is to reduce the volume of compost available for plant roots. If you’re worried about poor drainage, far better simply to add coarse grit or vermiculite to the compost.

What brings tears to my eyes is the number of perfectly good pots that must have been smashed over the years in the mistaken pursuit of improved drainage.

Drought – not all bad news

I’m not going to waste my time trying to convince you that a drought is exactly a good thing, and clearly a proper, 1976-style drought is very bad news indeed, for all kinds of reasons. But purely from a gardening perspective, a bit less rain than usual is not all bad news. So on the principle that one should always look on the bright side, let me give you three reasons to welcome another fine, sunny day.

First of all, put your hand up if you really enjoy mowing the lawn. I thought so – not too many of you. Few things cause more angst than a brown lawn, but a dry lawn grows less than a wet one, and a brown lawn doesn’t grow at all. Brown grass is not a sign that your lawn is dead, it’s just the grass’s natural way of shutting down until it rains again. Your grass will recover when it rains, and there is never any need to water an established lawn. And if you don’t believe me, this is the official advice from the Turfgrass Growers Association (www.turfgrass.co.uk), who grow 70 per cent of the UK’s turf between them, so they should know. In fact, watering your lawn during a drought does more harm than good. It encourages weeds and diseases, makes your soil more easily damaged by trampling, and encourages surface rooting, which means watering now makes your lawn more likely to suffer from drought in the future.

If you don’t want your lawn to go brown in the first place, here are a few other things you can do: (1) keep your mower blades sharp; (2) apply an occasional light top-dressing of soil or compost; (3) don’t use fertiliser: if your lawn isn’t growing, it doesn’t need feeding; (4) remove thatch with a lawn rake; and most important of all, (5) increase mowing height: taller grass has deeper roots, and also helps to shade and cool the ground. In time of drought, don’t cut lower than 35–40 mm. Newly laid lawns do need watering, but only for the first month – after that they should be able to look after themselves.

Weeds need water just like other plants, and annual weeds, which need water for their seeds to germinate, are particularly dependent on rainfall. So less rain means fewer seedlings of hairy bittercress, groundsel, chickweed and annual meadow grass. Many perennial weeds rely mainly on seeds to get around too: think dandelions, willowherbs, docks, pearlwort and plantains. Not only do all these plants need water to germinate, wet soil makes their seedlings much harder to kill. There are few more unrewarding pastimes than hoeing seedlings of annual meadow grass on a wet day, only to return days later to find that most of them are still happily growing and flowering away. In dry weather there will be many fewer weed seedlings to start with, and when you kill them, they will stay dead.

Last but far from least, less rain means fewer slugs, or at any rate lower slug activity. In spring, when seeds are germinating and new shoots are just emerging, plants are at their most vulnerable to slugs. In fact slugs will eat seedlings of plants that they wouldn’t (or couldn’t) look at twice as mature plants. Even tree seedlings are in danger from slugs; in one Swedish study slugs wiped out over two-thirds of Scots pine seedlings, but only if it rained – in dry weather few seedlings were attacked.

The lesson is to water seedlings, young plants and anything newly planted, but to do so carefully, applying the water only where it’s needed. Just spraying it everywhere will only attract slugs and persuade more weeds to germinate. Also try to water in the morning so that surface soil is dry by evening, which will discourage slugs. A useful discipline is to use a watering can – it’s good practice for when your water company imposes a hosepipe ban, and the exercise will do you good.

Flooding – leave well alone

Whether it’s flooded or not, it’s a safe bet your garden is basically sodden,¹ and as far as your plants are concerned, the effect is much the same. A typical soil is about 50 per cent empty space, and most of the time, most of that space is filled with air. The space in soil may fill up briefly after rain, but the water quickly drains away, at least from the larger pores and channels. Which is just as well, since roots need air to breathe and, if the spaces in soil become filled with water for any length of time, they start to run out of oxygen. If that goes on for too long...

Table of contents

• Copyright Page
• Contents
• About the author
• Introduction
• Garden wildlife
• Native and alien plants – and animals
• Not worth doing
• Names of plants – and people
• Growing food
• Interesting things about plants
• Practical gardening
• On being a gardener