Techniques for the cultural control of weeds work by improving the ability of our target crops (ie our veggies) to compete more effectively with weed species.
While cultural control of weeds is more often used in broad scale farming, it has techniques which can be used by the urban veggie grower to help their fight against weeds as well. Some cultural controls like mulching, solarisation and direct watering have already been covered in other parts of this series. Other cultural management techniques like summer tillage, stale seedbed and blind tillage are more suited to broad scale farming than the backyard organic grower.
There are, however a number of techniques for cultural weed control that can be implemented on a small scale in our backyard veggie growing efforts to help reduce weed problems. While these methods are unlikely to control weeds by themselves, when integrated with other weed control strategies mentioned previously they can be effective in reducing our weed problems in the long term.
Cultural control methods
• Use high quality, fresh seeds - they are more likely to produce vigorous and competitive plants. There also some evidence to suggest that varieties with larger, plumper seeds will germinate and grow more quickly.
• Plant well grown seedlings rather than direct sowing seeds – This is won’t work for root crops but will work for everything else. We do this by germinating our seeds in punnets, then potting the seedlings on into newspaper pots, growing them on and then planting several weeks, up to a month after germination, newspaper pot and all.
• Growing plants closely together – As well as using higher seeding rates, vegetable seedlings can be grown more closely in rich organic soil, which has the effect of shading out weeds that have germinated as well as reducing weed germination by reducing light. The same caveats apply as above for close seeding, we plant our veggies on a grid, 20cm to 30cm away from the plants adjacent to them and this works well for us.
• Where direct seeding is required use increased seeding rates - and narrow row spacing if you plant in rows. While this technique will allow the veggies to outcompete the weeds for light, air and nutrients, they will also compete with each other, so some research may have to be done to find the ideal spacing for each crop. Also, where this technique is used for crops like carrots, they can be sown thickly, and then as they grow they can be thinned strategically leaving room for larger carrots to grow, and the thinnings eaten as baby carrots
• Maintaining good soil fertility – some weeds such as wild carrot, dandelion, thistle and plantain are pioneer species and thrive in poor soil, maintaining soil fertility organically will reduce the impact that these weeds will have. These soil fertility techniques include adding compost, adding other soil amendments like wood ash, biochar or rock dust, using cover crops/green manures, crop rotation and inter-planting. Some of these techniques help to maintain fertility but also impact on weeds in their own right.
Well grown green manure crop
Green maure crop cut before flowering
• Cover crops/green manures – This technique involves growing plants to cover open ground which then outcompete and suppress weeds, shade the ground to reduce weed germination. Cover crops and green manures can also be used to add nitrogen and organic matter to the soil by sowing nitrogen fixing species, cutting them down before flowering and incorporating them into the soil. Nitrogen fixing species that can be used include peas, clovers or vetches, non-nitrogen fixing species include millet, mustard, wheat and radish. It is important to select species and varieties that suit your climate and the time of year they are being sown. More details on our use of green manure is available here.
• Use of ‘cleaning’ crops – This is a specific type of cover crop or green manure used to ‘clean’ the soil of weeds and soil borne diseases. The idea to sow mustard seed thickly as part of a rotation, let it grow, cut it down before it flowers, and water the area. As the mustard plants break down they release compounds called glucosinolates which in turn release a gas, isothiocyanate. This acts as a soil fumigant killing plant disease organisms in the soil (and I suspect some of the soil biota as well) and weed seeds. I have not tried this but there is enough information out there to suggest it could be an interesting technique to add to the repertoire. Of course, mustard being a brassica, brassicas should not be grown on land so treated as part of the next rotation.
• Crop rotation – crop rotation has been used as part of cultural weed management in broad acre growing working on the idea that growing the same crops over and over tends to make weed infestations worse. This is less of a problem in the backyard but crop rotation can still help with weed suppression, particularly if used in association with close planting and mulching. There are a number of rotation systems out there, a common 4 year rotation starting with potatoes and possibly a green manure to suppress weeds, followed by peas and beans to replace nitrogen, followed by brassicas and leaf crops to use the nitrogen, followed by root crops and alliums then back to potatoes. Another strategy is to not plant from the same vegetable family, two seasons in a row. Crop rotation is also is good for making things hard for plant pests and diseases. More information on how we manage crop rotation
• Use of livestock – in the farming world this would be grazing with goats, sheep, cattle or pigs but as this is difficult (not to say disconcerting for your neighbours) in the suburbs, the most logical candidates are chooks or rabbits (perhaps even guinea pigs!) We include chooks in our rotation so that as part of a chook tractor such that they change veggie plots every two weeks. They are great for digging up and eating weeds, increasing fertility with their manure and digging in any remaining straw mulch into the soil.
• Interplanting – also referred to intercropping in farming circles, it is simply growing two or more crops together, as opposed to monoculture, which is growing one crop in a given productive area. An associated technique is ‘undersowing’ where a second crop is sown under a crop which is already growing on the land. These techniques are particularly applicable where no mulch is being used. The interplanted and undersown crops will shade and outcompete germinating weeds seeds for light, water and nutrients. For example, where a slow growing crop like cauliflower or tomatoes is being grown, interplanting a quicker growing crop like radishes or lettuce will shade out the weeds while still producing a yield. Also, In the ‘three sisters’ planting technique, corn is interplanted with squash and beans, the beans providing nitrogen for the corn (as well as producing beans) and the squash shading and competing with weeds (as well as producing squash).
As mentioned previously, using cultural techniques will not control weeds on their own, but if we are growing organically we should implementing as many of these techniques as practical anyway. Not only with they help us control weeds but have associated with them many ‘side’ benefits like maintaining soil fertility, controlling pest and diseases and increasing yields from a given plot of ground, So it can be seen that it is important to make room for as many of these techniques as possible, when developing your weed control strategy.
