In the 2002 survey of weeds in field crops in Manitoba, dandelion was ranked as the 9th most abundant weed species and the 2nd most abundant perennial weed species. Between 1997 and 2002 dandelion's ranking among troublesome farm weeds rose by 13 positions. Driving across Manitoba in the spring of 2003 it was not difficult to find significant infestations of dandelion in field crops. Over the past 5 years many weed scientists have been asking why dandelion has become such a prominent weed pest in field crops. Their questions have lead to research and the results of this research is helping both farmers and gardeners to better manage this weed.
How Dandelion Spreads
One of the first questions we asked about the rise in dandelion's abundance is: How does it spread so successfully? We know that dandelion is a simple perennial (regenerates from rootstock but does not spread by vegetative structures) and so the spread of a dandelion infestation is dependent upon seed movement and seedling establishment. Greenhouse and field studies (Boyd and Van Acker 2003) have shown us that dandelion seedling emergence is favoured in moist soils and recent years of above average precipitation may be contributing to the rise in abundance of this species. We have also been tracking the emergence timing for dandelion seedlings (true seedlings from seed, not regenerating rootstock or over-wintering rosettes) and have found that the true seedlings do not appear until early summer (mid to late June - see figure 1). This means that true seedlings (the means by which dandelion infestations spread) only emerge after the period when we normally apply weed control techniques (herbicide or physical removal) either in farmers fields or lawns and gardens. Our weed control techniques target the over-wintering rosettes and shoots regenerating from established rootstock and not the true seedlings, so our normal efforts are doing little to prevent the spread of dandelion.
Figure 1. Emergence timing of dandelion shoots emerging from rootstock versus true seedlings emerging from seed. Data represents mean from 3 experimental site-years (2003) on wheat fields in southern Manitoba. Data from K. Hacault and R. Van Acker, unpublished.
Timing of Control
So when should we be controlling dandelion? Dandelion seedlings emerge in late June and a significant proportion of these seedling become fall rosettes which over-winter to produce tough to control shoots the following spring. Dandelion shoots have shown remarkable tolerance to herbicide applications in the early spring, especially if they are applied when there is still a risk of frost at night (Froese and Van Acker 2003). Dandelions are much more susceptible to herbicides applied in the fall (early September) (Froese et al. 2004). In addition, fall herbicide applications control the spread of dandelion infestations because they control those true seedlings which emerged in early summer of that year (the seedlings which likely emerged after spring control efforts were applied). For those who are not keen on using herbicides, physical removal of dandelions by spade, dandelion fork or tillage is also an effective means of management. In fact, we found that a single physical removal of dandelion top growth in the early spring (via tillage or dandelion fork) reduced dandelion regrowth the following spring by 50% on average (Froese et al. 2004). However, if physical removal is the management mode of choice, it must be done in the fall as well as spring if one is hoping to prevent a given dandelion infestation from spreading.
An Ongoing Battle
With the increased abundance of dandelion in farm fields many farmers have been asking whether their new dandelion infestation means a lifetime of management (many gardeners would likely ask the same question). The good news in this regard is that dandelion seeds do not exhibit long dormancy or longevity. The bad news is that dandelion rootstock does exhibit significant longevity, especially because the roots go deep. In addition, although dandelion doesn't produce a long lived seedbank, dandelion seed is spread very effectively over relatively long distances (100-500 metres) by wind. So limiting seed return in your own infestation does not mean that new seed will not reach your yard (or field) from someone else's seed setting infestation. Constant vigilance (and monitoring) is required for effective dandelion management.
- Dandelion spreads via the success of its seedlings and it persists via the success of established rootstocks. A management approach that recognizes these aspects of dandelion's biology will allow for better control.
- Neglecting to manage dandelion in the fall will lead to increasing problems. Fall management may not be sufficient for managing this species but it is essential.
- We can move towards taming infestations of dandelion but it is unlikely that we will eliminate them.
This page created August 2004.