Here’s the thing – we can’t completely ignore the hoar/whore leaves in the line before, alongside these long purples. I mean, why on earth is Gertrude bringing up a flower that looks like a cock? I mean, sure, maybe there were some long purples AKA Dead Men’s Fingers AKA some word for cock used by shepherds in these fantastic garlands. But Gertrude doesn’t need to reference them and if she does need to mention them, she doesn’t need to reference their more “liberal” name. It’s like she’s asking us to picture purple dicks and then asking us to imagine dead men with fingers like purple dicks. It’s WEIRD. Gertrude doesn’t make any other sexual allusions the whole play and here in her recounting of Ophelia’s death, she’s gone with two, in the first two lines of the speech. It is really mysterious.
Every Gertrude I’ve ever seen (including myself) just rides through these references, just puts her shoulder to the wheel of her most dignified Queen face and leans on the rest of the poetry.
But I’m long past an era of wanting to do things correctly and appropriately. Now, I’d like to lean into this weirdness, to make it as odd and out of joint as it seems, to not smooth over the strange choice to make a dick joke in the middle of a poetic death speech.
Or what if Gertrude came in all muddy and covered in flowers herself? What if she witnessed this and tried to save Ophelia and she has one of these garlands in her hand and just looking sat those long purples makes her laugh? I’d be interested in a production that made that choice – to watch her almost lose it – and then pull it together for the rest of the play.
The Hamlet Project 
What I’m about to suggest is just a little theory as to what’s really going on here but it’s based on two implicit assumptions
1. The canon of Shakespeare was a royal and aristocratic enterprise
2. The plays were not written for public audience
I agree with your sentence that it’s so utterly bizarre to make a dick joke in the middle of a poetic death scene and even more bizarre for that joke to come out of the mouth of a queen, and for the circumstance to be when this queen is breaking the news of that death of the daughter of her chief minister to the girl’s brother, and this being the very same girl that she possibly wished to see married to her own son, the prince.
To me the most probable way this sentence makes sense, as with many odd and unusual sentences in the canon, and especially in the script of Hamlet, for example another very very odd reference being ‘as by Lot God wot’, is that these bizarre references were meant for insiders who knew exactly what the reference was being made to.
It’s utter nonsense in my opinion that these plays were meant for us common folk. Charlie Chaplin and Buster Keaton are meant for us common folk. This stuff was all meant for the aristocracy, and not just any aristocracy but the very creme de la creme, that select band of wolfish whores and inbred bastards that was the royal court.
And to place a reference like that to that particular flower in the middle of that scene has the maximum shock value for the specific particular individual or individuals in the private royal audience that it was intended for.
Within the Virgin Queen’s royal inner circle, something sexual had happened with that particular flower or a reference to it as being cock like and this was a reference to that event
That’s my opinion and this particular reference for me epitomises absolutely everything to eyes that see about what the works of Shakespeare really are.
Well done for commenting on this. All the best and Merry Christmas