Use of agg. in plant taxonomy

Is there a simple explanation anyone can give me regarding the choice of, for example, Dipsacum fullonum versus Dipsacum fullonum agg. as I’m not getting very far with an online search for an answer. Many thanks.

An aggregate is a group of taxa that are difficult (even for botanists) to tell apart. In this case, I believe that the Dipsacus fullonum agg. consists of Dipsacus fullonum and Dipsacus sativus. According to Stace Dipsacus sativus differs in having shorter (as long as the flowers) bracts with rigid (not flexible) recurved (not straight) apical spines. It occurs, inter alia, as a bird seed alien.

Thanks, lavateraguy. Would you advise putting ‘teasel’ observations under Dipsacus fullonum agg. in that case? I germinated a couple of purchased teasel seeds some years ago and these continue to self seed.

Whether I’m right or not, I just put them all down as Dispacus fullonum (except for a very few observations of Dipsacus laciniatus or Dipsacus pilosus). On the other hand the Cheshire recording sheet has Dipsacus fullonum agg. rather than Dipsacus fullonum.

Looking at the BSBI database, it has 138 records for Dipsacus sativus (only 10 this century), 67292 for Dipsacus fullonum (30305) this century, and 80680 for Dipsacus fullonum s.l. (equivalent to Dipsacus fullonum agg. - the two taxa have been treated as forms of a single species at times) implying 12000+ records where the recorders either didn’t make a call to the species, or where their intent was ambiguous. (There are 12 records of the hybrid between the two species.)

So, it seems that while there is a possibility of Dipsacus sativus being overlooked at times, it’s rare enough that it’s not unreasonable to discount it. If teasels are naturalised on your property you do have the opportunity to take a closer look at the bracts when flowering.

the mess
perhaps you need to analyse a few of the Observations. Begin here
The drop-down is misleading. You and WE need to see as many Other Observations in your Ob. as possible. Agg. Obs are not seen with Species ones. Genus would collect them ALL - I think that might be valuable in Other Observations displays.
Perhaps check to see which is the most ‘popular’ ID and why. Say why you have chosen the Agg. (if you do) and perhaps help set the scene for others who follow.
Make your own decisions of course but I ran into this issue with Dandelion (Taraxacum). I am trying to get people to use the Genus ONLY so that they all appear together. Dandelion is a slightly different case.

If I was to ‘do’ a iSpotlight I MIGHT consider making a plea to ID to Genus ONLY and add a note about the difficulty - if it arises.

Nice and useful response, thanks

Many thanks for your time on this with all the details and the encouragement to check the bracts - it will be in July for me.

Thanks for this, Derek and the comparison with Dandelion. I’ll be checking the various different observations out.