Dear Allen

for detecting herbicides using fast blue BB dye were reported to be successful. 

for more information take a look at attached pdf

Acid blue 9 and Acid red 73 have been shown the optimal choice, with an extended reference to marketed dyes to this purpose by Anna Brown et al in Forestry vol76 n4 371- 

I have used Rhodamine Wt for quantification using a fluorimeter. This was both herbicide and insecticide research.

There is a product called Vision Pink by Garrco Products Inc.

Here is the Anna Brown article: https://www.researchgate.net/publication/31377316_The_use_of_dye_markers_as_a_potential_method_of_reducing_pesticide_use

If you illuminate the plants with UV light source you might get better contrast with some dyes. Fluorescent dyes might degrade too rapidly?

Article The use of dye markers as a potential method of reducing pesticide use

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Hi Alan

These some useful references 1

https://www.scientificamerican.com/article/bring-science-home-capillary-action-plant/

2

https://nopr.niscair.res.in/bitstream/123456789/13338/3/IJNPR%202(4)%20403-408.pdf

3

https://pioneerthinking.com/natural-dyes

4

https://www.gardeningknowhow.com/special/children/natural-plant-dyes.htm

5

https://www.bestdye.com/dyes-uses.php

6

http://homeguides.sfgate.com/food-dye-affect-color-plant-102401.html

7

http://homeguides.sfgate.com/effects-food-dye-flowers-roots-101292.html

8

https://sustainabilityforschools.org/assets/chemistry-guide-for-teachers.pdf

9

https://www.nuffieldfoundation.org/sites/default/files/14_Extract_test_plantdye.pdf

10

http://www.penguinrandomhouse.com/books/530292/natural-color-by-sasha-duerr/9781607749363/

11

https://detoxinista.com/2012/08/6-things-you-should-know-before-using-henna-hair-dye/

12

https://books.google.com/books?isbn=1439851867

13

https://books.google.com/books?isbn=0824806980

14

https://nptel.ac.in/courses/116104046/26.pdf

15

https://books.google.com/books?isbn=0824806980

16

https://sensing.konicaminolta.us/2015/07/can-colored-lights-affect-how-plants-grow/

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17

https://nptel.ac.in/courses/116104046/26.pdf

18

https://books.google.com/books?isbn=1933039744

19

https://sensing.konicaminolta.us/2015/07/can-colored-lights-affect-how-plants-grow/

20

https://www.ijsrp.org/research-paper-1014/ijsrp-p3484.pdf