Most agricultural weeds germinate and thrive on bare or open soils. They are the species of disturbance habitats and real pioneers. As a result they can not survive in vegetations composed of perennials. As a rule they require relatively rich, (moderately) dry and warm soils. If one can provide these conditions, many species are available for application in urban gardens and parks. One must start, however, with a soil that is free from persistent unwanted weeds. These are usually biennials or perennials, such as Tussilago farfara, Polygonum amphibium (land form), Elymus repens and Cirsium arvense. It may be necessary not to start sowing the desired species in the first year, but to take this period to cultivate the plot repeatedly until it is thoroughly cleaned of unwanted weeds.
Although the desired species used to occur in fields with agricultural crops, such as cereals, they are not restricted to these. Therefore, it is not necessary to sow the crops as well. It is not so much the presence of the cultivated crop as the disturbance habitat the field presents that provides the precondition for their survival. Depending on what one wishes to achieve, one can, of course, sow the cereals as well; species like rye can be very beautiful and, for educational purposes, it is often indispensable. If one chooses to do so, a quantity of 40-50 kg/ha will suffice.
Sowing is done by hand, in a wide sweeping motion. Mixing the fine seeds with sand helps to distribute them more evenly over the sowing area. The required seed quantity is determined by the size of the plot. Larger plots require less seed per square metre than smaller ones, where low density is less visually acceptable. Another important factor is seed size: Papaver has very fine seeds, whilst Agrostemma has coarse and heavy seeds. As a general rule, one requires 1-3 g/m2. After sowing the seeds they must be lightly worked into the soil with a rake or harrow, depending on the plot size. Subsequently the soil is slightly compacted using treading boards or a roller.
One has the choice of either composing a seed mixture of different species, or sowing each species separately. This way one can create all sorts of compositions, for aesthetic or agricultural-historical or ecological reasons. If one prefers an aesthetic starting point, one could use, for example, Papaver rhoeas with Matricaria recutita, Papaver rhoeas with Chrysanthemum segetum or both combined with Centaurea cyanus, Chrysanthemum segetum with Delphinium consolida, or all species put together with Agrostemma githago. Vaccaria pyramidata is, especially in warm dry springs, a fast germinating and growing species whose flowering period usually precedes that of other species. By sowing a slightly larger quantity of this species, one obtains an early peak of flowering, followed by a second one when the other species are flowering.
Most species can be sown in spring, from the beginning of March until the end of May. March is preferable, since germination is less successful in later periods due to cold and dry spells caused by the April north winds. The seeds of unwanted species already present in the soil are less susceptible to these climatic disadvantages and thus get a head start on the others, causing aesthetically less acceptable results. Some species germinate better after an autumn sowing, for example Legousia speculum-veneris and L. hybrida, Ranunculus arvensis, Galeopsis segetum and G. speciosa, and Scandix pectenveneris. If winter cereals are included, one obviously has to sow in autumn. The best way to go about this is to finish sowing before the middle of October. The grains must have germinated before the first frost arrives. Spring sowing is generally preferable for soils containing a lot of weeds. Since they germinate early, one can remove them by working the soil over before sowing the desired species.
