2 Celtic fields and rye cultivation
The oldest finds of rye in the Netherlands, NW Germany and Denmark date from the Late Iron Age/Roman period, and probably represent a weed grown in fields with other cereals. The start of rye cultivation in NW Europe has to be placed later, in the first centuries AD, with a rapid expansion in the Migration/Early Medieval period, and becoming most intensive in the 2nd millennium AD (Behre 1992, 2000; Behre & Kučan 1994; Mikkelsen 2003).
In the grain finds of the middle of the first century, rye is equal in numbers to or even exceeds other cereals (Behre & Kučan 1994, 39, tab. 6). It is assumed to have been grown as a summer crop. Summer rye should produce less pollen than winter rye (see also Behre & Kučan 1994, 36-41, 57-62, 69-70, 85-87).
Around AD 1000 there is a strong increase of rye in the pollen diagrams in NW Germany, together with an explosion of Rumex acetosella (sheep’s sorrel), one of the most important weeds of rye fields. Typical weeds of winter rye fields, such as Agrostemma githago (corn cockle) and Centaurea cyanus (cornflower) follow in the course of time. Mikkelsen (2003, 152-154) summarizes the introduction of rye in Denmark, already starting in the pre-Roman Iron Age (2nd and 1st c. BC), again probably as weeds in cornfields. From the middle of the first millennium onwards there are numerous finds of carbonized rye grains, here supposed to be of winter crops, because of a much earlier rise in Rumex acetosella in Danish pollen diagrams. However, this species is furthered by, but not dependent on, rye and may also occur in abundance in sandy grassy areas. Moreover, the similarity between weeds in summer and winter corn fields in the same habitat is great, the difference lying mainly in the cover percentages (Haveman et al. 1998, 206). Pollen data from Western Jutland, show low values for Cerealia around the beginning of our era, the start of rye cultivation in Jutland around AD 300, gradually increasing in the second half of the first millennium (Odgaard 1994, 155-157; 2010).
Rye pollen data from archaeological sites in the central part of the Netherlands show a slight increase from the Roman period to the 12th century and from then on a steep increase (fig. 1) (Groenman-van Waateringe 2010, fig. 3.9). The latter is in accordance with Behre’s ideas on the link between winter rye cultivation and the development of plaggen soils.