When choosing your organic slug and snail defences, the big question you need to ask yourself as an organic gardener is how do you feel about slugs and snails? There are a spectrum of options depending on your attitude to our slimy friends:
A. Slugs and snails are my nemesis. Death to molluscs!
1. Stamp on them
You're most likely to find them when it's been raining or at night (for instance with a head torch). The method's as simple as it sounds. Find them and stamp on them. You can just leave them crushed on the ground. Something - I suspect birds or hedgehogs - will come along and eat them later.
2. Drown them
Wear gardening gloves (slugs feel as disgusting as they look). Fill a bucket (or similar container) with water and a bit of washing up liquid. Go round the garden gathering every slug or snail you see and popping them into the bucket (good times are after it's been raining or after dark with a torch). Job done.
3. The Black Plastic Bag Method
Place a black bin liner on the ground between your tender plants. Put two lettuces that are too old (or too pest-eaten) to eat in the bag, add two teacups of breakfast bran and a full cup of beer (ideally bitter). Leave over night with the top open. Check for slugs in the morning and either drown them or take them a very long way away and release them (be warned, slugs and snails seem to have quite a good sense of geography and may succeed in coming back to your garden unless you take them a really long way away - no one seems to be completely certain how far - I'm tempted to look for empirical evidence with snails and nail varnish marks).
4. The grapefruit method
Halve a grapefruit and remove the flesh (I suggest by eating it for breakfast).Put the rind on the ground in the garden near your tender plants, hollowed-out side down. Leave it out overnight. Next morning, check underneath for slugs and snails and dispose of them by one of the options described in the black plastic bag method.
B. I want them dead, but I don't want to do it myself, killing them directly makes me feel guilty.
5. Beer traps
Put some beer (or 2 cups of warm water, 2 tablespoons of flour, 1 teaspoon of sugar and ½ teaspoon of yeast) in a smooth-edged container in the ground. A plastic cup or yoghurt pot will do, but for a longer lasting slug trap that can withstand both rain and evaporation, try turning a plastic bottle into one of these:
You then need to bury them on their side at an angle in the soil so the slugs can crawl through the bottle neck into the beer at the bottom. The problem with these is that only slugs and smaller snails can get in. You need a bottle with much bigger neck for full-size snails to be able to get in and drown too. Apparently it's not actually a problem leaving rotting slugs and snails in them. My slug traps probably stopped working because the beer got watered down by rain and didn't smell strongly enough any more, not because of the dead molluscs floating in them. When you do finally empty them, you can just tip the slugs and snails out onto your grass or soil for birds to eat.
6. Nematodes
Buy these from a garden shop or online shop. Apply early in the growing season (but after the soil is above 5°C (40°F)). Follow the instructions on the pack.
7. Encourage predators
It's not just the French who eat snails. Others who consider snails a suitable snack (and are equally happy to munch on slugs) include ground and rove beetles, centipedes (remember them on the list of friends in the garden pests post? now you know why), frogs, toads, slow worms, hedgehogs and many species of bird.
7a. Frogs and toads
Keep a pond (without goldfish).
7b. Slow worms
Leave areas of long grass.
7c. Hedgehogs
Put dog food out at night (not bread and milk, that's bad for them) and provide them with a waterproof box to live in. The box should be about 45 cm (18 inches) long and 30 cm (12 inches) deep. Fill it with straw to make it nice and snug, then hide it by covering it with leaves.
7d. Ground and rover beetles (the trench method)
Dig a trench about 15 cm (6 inches) deep and 7.5 cm (3 inches across). Line it with pebbles and Perspex edges. Beetles will fall into the trench and can hide under the pebbles, which will protect them from birds. When a slug falls in, the beetle will eat it for dinner.
8. Keep your soil slug-unfriendly
Create a fine tilth on the top of the soil (fine, crumbly soil created by digging and raking). Also, if you only object to killing sentient slugs and snails, rake your soil in the winter, as this will expose slug eggs to frost damage.
9. Tidy up any dead plants or weeds or fallen leaves
Slugs eat decaying vegetation, so this is a source of food for them and will encourage them. Remove it (and put it in your compost heap).
10. Vermiculite
Vermiculite is a type of mineral which swells up when in contact with water. It also moves about in soil, which slugs apparently dislike. Add it to compost and put it around plants, especially those which like water. This puts slugs off. It is allegedly 90% effective.
11. Copper
Put copper slug rings round your plants or copper tape around your pots or raised beds. Make absolutely certain there's no gap between the slug ring and the soil, not even a tiny one, baby slugs and snails are really tiny and will slip through tiny gaps. Also, make sure there aren't any bridges over the slug ring or tape that would allow slugs or snails to climb over it (e.g. leaves hanging over the edge). Also make sure no slugs or snails or their eggs are already inside. Slugs and snails may also find ways to lower themselves in from overhanging plants, so none of those - they're surprisingly able to find their way from A to B, but if the only option is to crawl across copper, they won't go there.
12. Other barrier methods
You can also make barriers out of baked egg shells or coffee grounds. I'm pretty sure I once watched an Alan Titchmarsh programme where he proved these all ineffective, but they still might be better than nothing.
A quick note on metaldehyde (conventional, non-organic slug pellets)
The most effective way that I know of to deal with slugs is not to be organic. Slug pellets containing metaldehyde kill slugs and snails quickly and efficiently. There are a few problems with it though. Firstly, it's a poison and is also poisonous to pets and children (or for that matter adults or other garden wildlife). There seems to be some disagreement as to whether it bioaccumulates (builds up in an organism because it's absorbed faster than the substance is lost). Our teacher on the gardening course says it does and gathers in animals higher up the food chain that have eaten poisoned slugs and snails. A quick search of the Internet says it doesn't. Either way, it's not organic, so if you want to garden organically, you should avoid it.
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