Weeds - friend or foe?
Carol, Carol's Garden
Why do photographers love taking pictures where the weeds are worst?
When are weeds a problem?
We all know that saying ‘a weed is just a plant in the wrong place’? If you are growing any crop, then any other plant which is interfering with that crop’s ability to grow well, or with your ability to harvest it, counts as a weed. Weeds can be a problem in the following ways:
Crowd out small or germinating plants - preventing light from reaching the leaves of smaller or less competitive plants. If you have existing weeds in a bed that you want to direct sow, you will need to remove them or they will almost always shade out the germinating crop. To see how effective this is, you only need to clear a bed of all plants, and leave for a few days to see how quickly seeds will germinate in the new light and space.
Unwanted plants may starve your crop of water or nutrients as well as light - Plants will grow to suit the space they have available. If there is lots of competition from neighbours, they will usually grow narrower and taller, maybe flower sooner, before they have reached full size. Sometimes this is desirable, so we plant densely, but sometimes it is not going to give a good crop over a long period.
They may harbour pests and diseases - lots of pests (and some helpful predators) will take cover in weeds around your crops - whether in the same bed or in a neighbouring bed, or in the hedge etc. Aphids can spread from weeds, Slugs & snails love damp shady soil under ground covering weeds, but so do ground beetles, which prey on them. Voles and mice like any kind of cover to hide in. If weeds are related to your crop species, they may carry diseases, such as rusts, even viruses.
They can make harvesting difficult or inconvenient! No-one likes working round thistles or nettles to cut flowers. Weeds tangled in with your crops can really slow you down when you are cutting. Weeds along paths can easily put you off - a clear path is easier, so you use that path to cut from instead, so weedy plants go uncut.
I think 6’ high fat hen may qualify as inconvenient now - time to cut it out!
Weeds as friends?
There are lots of times when ‘weeds’ as we generally know them, can be tolerated or welcomed:
They can provide cover for exposed soil and shade for vulnerable seedlings and transplants in hot dry conditions. I often welcome chickweed proliferating under established plants in the polytunnel or outside. The soil is always damp underneath and the chickweed does not seem to compete with bigger growing plants e.g. dahlias, cosmos. I allow Fat Hen (Atriplex) to grow in some of our beds in spring, to provide some shade in the heat, and just make sure I remove it before it gets too big or sheds seeds.
Some crops can tolerate weeds without causing any or much loss of productivity eg trees, shrubs, vigorous perennials (my mint bed is full of comfrey and nettles, but they coexist ok). The time and effort required to try to remove them will not be repaid by any significant increase in productivity
They often provide valuable biodiversity and harbour beneficial species, such as wasps, ladybirds, beetles etc etc. Areas of weeds or naturalised plants are often a positive benefit and can be allowed to grow, perhaps in isolated areas if there is a risk of them outcompeting your crops.
Some of them can be cut and sold! Either in leaf, flower or seed head. eg Dock, bracken, willow herb.
Main methods of weed control and reduction:
Black plastic mulch
Mulching:
There are various advantages of mulching, but when we are looking at weed control, the main purpose is to exclude light from the leaves of growing plants or from seeds waiting to germinate. Photosynthesising plants cannot survive without light. So, the more effective a mulch is at excluding light, the more effective it will be at controlling weeds. In US, the process of excluding light is called occultation. Mulches you can use:
Opaque (usually black) plastic mulches inc weed-suppressing membranes — these are the most effective at excluding light. Woven membranes do have tiny holes and some persistent weeds occasionally grow through them - but most often from a seed rooting down through them rather than a shoot coming up. The advantages are that they are very effective at weed control by excluding light, whether used to kill off weeds before taking off the plastic and then planting, or for planting through holes in the membrane. Black plastic sheets don't allow water through, but are good for keeping areas weed free before panting. Disadvantages are that they are plastic (but can be reused many many times), they can harbour pests (slugs, voles), can make watering harder, more work to install and to plant through. There are some single-season plastic and plant starch-based mulches used in large field scale systems, usually needing a tractor-mounted implements to lay.
Organic mulches - These only work in reducing weeds if they are weed free to start with. Most home made compost and manures are full of weed seeds, so, although beneficial in many ways, will not help to keep weeds down! Municipal compost, straw, Spent mushroom compost, spent hops, seaweed etc are all relatively free of weeds and will help to reduce seed germination. They will not stop established weeds from regrowing, but a ‘blinding’ layer of cardboard, newspaper etc on top of soil or weeds will help to do this. They will not, generally be persistent or complete enough to kill off established perennial weeds, unless the compost layer on top is very thick (min 6”). This technique of using a blinding layer of cardboard or newspaper under a thick layer of organic compost is at the heart of ‘No-Dig’ growing.
Bark, gravels, carpet etc will help to shade out emerging weeds for a while, but without a weed suppressing membrane layer, most will ‘disappear’ into soil and the light barrier is lost over time. And weed seeds will germinate into them.
Weed suppressing membrane as a path, with straw mulch between Achillea plants. This is at the end of the season, before any weeding or re-mulching. You can see that the straw has decomposed a bit, but there aren’t many weeds, and none which are causing a real problem. The bed was weeded and mulched in spring.
Hoeing:
This is great for killing off small seedlings in exposed soil. The most efficient way is to hoe often, almost before seedlings appear - disturbing them before they have chance to get established. Surface hoeing on a hot day will be relatively quick and very effective. There are various hand hoes available, the best for you will depend on your soil and your preference. Wheeled hoes are good only if you grow in long lines or long beds. Here's a video of Paula showing us hoeing methods.
Hand weeding and digging them out:
To be avoided where possible - as its slow, backbreaking and you end up sending lots of soil into the compost bin. But there are times when it has to be done - but do try to mulch over any exposed soil afterwards
Reduce seeding:
1 year’s seeds: 7 year’s weeds!! We don't weed much at all in summer - but we will go through cutting off any big seed heads before they drop seed everywhere. This is much quicker and can reduce the seeding problem significantly.
Mowing or close strimming:
This will control annual weeds, and continually cutting the tops off perennial weeds will, eventually, kill them off too. Think of lawns. Even docks will not survive being mowed off to 2” every week.
Spraying:
You may want to use herbicide sprays to control weeds. If you are going to do this, you need to read the instructions and to follow them carefully and responsibly. If you are doing this for your business, you may need to have a professional certification (in the UK, this is a PA1 or PA6 certicfication) or employ someone who has. Please ensure you are very clear about any impact on the wider environment as well as on your intended target.
Know your enemy
Fat hen weeds
If you know what weeds you have, you can deal with them accordingly.
Seedling weeds:
These are any weeds which will spring up as soon as you leave any soil bare. These will include fat hen, annual nettle, chickweed, thistles, as well as all those hardy annuals which seed themselves merrily where you don't want them!
Solutions for these are hoeing, (as soon as they emerge); mulching of any kind (thick weed free organic mulch is often enough to exclude light and prevent germination), hand weeding if they are large eg sycamore seedlings.
Established annual weeds:
Mature plants of fat hen, chickweed, annual nettle, poppies in the wrong place
Solutions for these are to hand weed, or at least remove seed heads before seeds drop. If they are in a bed with no crop or a finished crop, they can be killed off with a light-excluding mulch eg where alliums have finished: trim off and cover with black plastic until early spring. There is absolutely no point treating established annual weeds with herbicide as they will seed before they die. there isn't much point hand weeding them all out as they will die off anyway - cutting the heads off will be sufficient.
These chickweed plants are bigger than seedlings, but not really causing a major problem to the foxgloves they are growing between. If I want to prevent them seeding, now is the time to weed them out. If I don’t mind the seeds, I can leave them and just pull out the bigger weeds - nettles and thistles.
Established perennial weeds:
eg Nettles, Docks, Brambles, Thistles, Bindweed, couch. These are much harder to eradicate.
Options for these include: Digging them out - usually by hand. You need to try and remove all bits of root as all these plants are vigourous and will regrow from small pieces of root; Mowing - if you have areas of these sort of weeds, continual close mowing for a year will eventually kill them off; Excluding light eg with black plastic or weed suppressing membrane - for at least a whole growing season. Herbicide - Might be a last resort for some of these.
The types of soil and conditions you have will strongly affect the types of weeds you have.
Our Worst 5 weeds:
Carol - Fat hen, willow herb, annual & perennial nettles, willow seedlings (because they get too big before I notice them!)
Paula - Docks, docks, docks, couch grass, nettles
Claire - Bindweed, nettles, fat hen, chickweed, thistles
Docks after 6 months under plastic mulch
Our Top weedy tips
Our comprehensive! combined experience of weeds, lets us give you the following tips
Don’t sow weeds - try not to use mulches or composts with lots of weed seeds in, if you can. Keeping weed seeds out of your own home made compost is difficult, but we do try to put problem weed seeds in a separate ‘bad weed pile’. We used quite a lot of wheat straw this year, but it did grow wheat. We weeded it out where it was a problem, but left it in other areas, to cut green and to dry.
Don’t plant weeds - this might sound daft, but there are often tiny weed seedlings already growing in modules and pots. Make sure you pull them out before you plant them.
Do try to create a ‘stale seed bed’ for direct sowing or for planting out small plants - this is where you prepare your bed for sowing, let the surface weed seeds germinate (or even encourage them with a clear plastic cover) and them kill them off eg by hoeing. This will reduce the quantity of viable seeds in the surface of the bed.
Do sow when it’s dry - and sow into a drill that you have already soaked. Cover with dry soil and keep the surface dry for as long as possible. If you need to water, water the drill only, not the rest of the soil surface in between.
Don’t mulch perennial weeds - a cosy layer of organic mulch without a ‘blinding’ layer of paper, cardboard or plastic will just encourage them to grow even bigger - they will love it!
Eradicating perennial weeds can be a long term ‘war of attrition’ - but don't forget that nothing much will survive constantly being chopped off at ground level, or permanently being deprived of light
Don't try to be totally weed-free. Just prioritise your efforts on those that are causing a real problem now, - or will be soon if you don't nip them in the bud.
.
A general point on no-dig growing:
This relies on using weed-free mulches, and on keeping surfaces relatively weed-free. All three of us use no-dig systems to varying degrees, so most of our general advice is based on hand scale growing, without digging or use of mechanical cultivators or rotavators. This is easier in veg growing systems than flower growing, as veg rarely grow to seeding stage. We often grow flowers to seedhead stage and so we get a much higher burden of ‘weed’ seeds on our soils and in our composts. By far the most common seeding ‘weeds’ here are poppies, nigella, grasses, honesty etc that we have left to form seed. Sometimes useful, sometimes just too many! You can find more resources on specific ‘No-Dig’ practices via many books and websites, particularly Charles Dowding who has written widely on this, although primarily for veg growing.
A general point on green manures:
Green manures and cover crops are sown to protect soil and to introduce nutrients and/or organic matter into soil. Usually sown in the autumn, sometimes to over winter. There are various different varieties that can be used, depending whether you want added nitrogen, organic bulk, or trace elements being captured from deeper rooting species. They are almost all incorporated in early spring, either by rotavating or digging in and so are not generally compatible with no-dig systems. Some are not hardy and will be killed by frost, leaving a clear bed. As well as their benefit to soils, they reduce weed pressure by outcompeting weed seeds. The drawback is the need to incorporate or eradicate the green manure itself. Trials are ongoing on the use of green manures without the need to dig them in, for example by ‘crimping’ or flattening the green manure and then covering with plastic sheet, or just planting young plants into the stems of the green manure. Choose your green manure variety carefully, or they can become a weed of their own.