Looks like I was a bit hasty with the translation of 'lis'. The 'flag' reference is apparently another name for an iris. Of course somebody could have been stood waving a bunch of irises instead of a flag during the daytime.
As for the flower, it is apparently used in water purification. It is also the symbol for Brussels and is the origin of the fleur de lys.
http://en.wikipedia.org/wiki/Saint-G%C3%A9ry_Island'Saint-Géry Island (French: île Saint-Géry, Dutch: About this sound Sint-Gorikseiland (help·info)) was the largest island in the Senne (Zenne) river in Brussels, Belgium. It was named after Saint Gaugericus of Cambrai, who built a chapel there ca. 580. Hence the name "Brussels", which comes from Bruocsella or Broekzele, meaning "settlement in the marsh".[1] It ceased to exist as an island when the Senne was covered over in the late 19th century.'
This island was apparently 'carpetted with irises'. 'The island was said to be once completely carpeted in irises. Due to the importance of the island, the iris has been the symbol of Brussels since the 19th century, and is now the sole feature on the flag of the Brussels-Capital Region.[2]'
Lilies can also be used for water purification removing heavy metals and dealing with sewage.
http://www.prnewswire.com/news-releases ... 21227.html'Water lilies may prove to be a secret weapon in the fight against water pollution. Recent experiments have shown
water lilies to be capable of absorbing great quantities of heavy, poisonous metals through their leaves and roots. This could allow water lilies to be used both to restore the purity of rivers and water sources polluted with
poisonous heavy metals and as a means for treating waste water originating from household or industrial sources.'
Irises are also used in aromatherapy and were in use in ancient times for a variety of medicines.
http://en.wikipedia.org/wiki/Iris_%28plant%29'Rhizomes of the German Iris (I. germanica) and Sweet Iris (I. pallida) are traded as orris root and are used in perfume and medicine, though more common in ancient times than today. Today Iris essential oil (absolute) from flowers are sometimes used in aromatherapy as sedative medicines. The dried rhizomes are also given whole to babies to help in teething. Gin brands such as Bombay Sapphire and Magellan Gin use orris root and sometimes iris flowers for flavor and color.
For orris root production, iris rhizomes are harvested, dried, and aged for up to 5 years. In this time, the fats and oils inside the roots undergo degradation and oxidation, which produces many fragrant compounds that are valuable in perfumery. The scent is said to be similar to violets. The aged rhizomes are steam-distilled which produces a thick oily compound, known in the perfume industry as "iris butter" or orris oil.'
It could well have been a good product to trade with.