If you wish to repeat this quest for multiple seeds, you might consider first befriending the colony on your facet. This is done via the Solen Matriarch Quest (which also provides Translocation Powder). Once completed, Solen within that hive will no longer attack you without provocation.
Speak to any Naturalist NPC to begin the quest (Mike is one example, though others can be found in various libraries). Make note of which Naturalist you visit, as you will need to return to the same one to report your findings.
The Naturalist will offer you an opportunity to complete a study of the Solen Hive. To complete the quest, you will need to visit each of four egg nests within (in any order). The storyline has you scribbling notes on your parchment, but in reality, you just need to stand quietly near the nests for about 30 seconds while the fiction plays out.
Examining The Nests
It is easiest to enter via the Minoc entrance. The monsters within are not overly fast and can easily be out distanced by those who cannot fight them. Once in the hive the path is fairly linear; simply follow passage until you find the first Egg Nest (easily recognised by it's distinctive egg pods).
When you reach the first nest, you will hear yourself exclaim "A-ha!" out loud. Stand still for half a minute or so. During this time you'll hear scribbling on parchment and see these messages on your screen:
"You begin studying the Solen Egg Nest to gather information."
"You begin recording your completed notes on a bit of parchment."
"You have completed your studies of this Solen Egg Nest. You put your notes away."
Once completed, continue to follow the passage deeper into the hive until you finally reach the center (where the Solen Matriarch is located). From here you have access to the three other tunnel branches, each of which contains a fresh nest for you to examine. Once you have visited all four you're ready to leave the hive.
The Secret Nest
At this point, you have a decision to make: You can return to the Naturalist that you visited initially and claim your reward, or visit the legendary secret nest. Whether or not you go the extra mile will have some hand in the type of seed you are rewarded with.
In order to find the fifth nest, you need to obtain a Green Thorn. Your local gardener may already have a supply of these, though you can always grow your own Bright Green Barrel Cacti / Bright Green Snake Plants if you have the patience.
Take the thorn to any sandy area (such as a beach or desert), double click it then target the ground. You will see a message that you've pushed the thorn into the sand, which will collapse shortly afterwards revealing a mysterious hole.
Jump into the hole by double-clicking on it and you'll be transported to the mysterious secret ant tunnel. Note that sometimes the hole won't be visible on your screen due to a glitch, but it's there: Just double click around the area where you planted the thorn.
Run left to the secret ant nest and study it as you did the others:
You notice something very odd about this Solen Egg Nest. You begin taking notes.
You complete your examination of this bizarre Egg Nest. The Naturalist will undoubtedly be quite interested in these notes!
Upon completion of your note taking, you'll be excited to bring your findings back to the Naturalist.
Head back to the Naturalist where you received your original quest instructions. Talk to him or her and you will receive your prize - a rare pink, magenta, or aqua seed for completing nests 1-4, or a black, white or fire red seed for completing all 5 nests.
These will either grow into the same plant types as the breedable varieties, or a peculiar variety. The resulting plants will not produce new seeds, but you can repeat this quest multiple times to earn more.