As a tertiary ingredient used in the Summoning skill, marigolds are always in demand. And with Farming being almost the only source of marigolds, the demand is not always met. Farming marigolds can thus be somewhat profitable for low-level players, as it requires only level 2 Farming
and is even available to non-members.
Members have access to four farming patches in which marigolds can be farmed - one near Falador, one near Port Phasmatys, one in Catherby and one at the Manor Farm. Only the one near Falador is available to free players, so if you're not a member you'll make roughly one quarter of the profits listed here. Apart from membership, none of these patches have any requirements to access, but the one by Port Phasmatys can be time-consuming to get to without the ectophial or a modified farmer's hat (the latter of which can only be used three times a day) - the easiest way to get there without either of those is teleporting to the Canifis lodestone, though skipping the patch might be a better option. The Falador patch also takes a little bit of time to reach without the Explorer's ring 3 or the South Falador Teleport spell, although the Draynor Village lodestone is reasonably close by. The Manor Farm and Catherby patches are easy to reach with the lodestone network, and should always be used.
Every five minutes while growing, marigolds have a risk of becoming diseased. When diseased, the marigolds have a roughly 50% risk of dying every 5 minutes and will not grow further unless cured using plant cure. As the frequency of disease isn't known, the profit calculation doesn't include this - actual profit may be somewhat lower than the listed number for this reason. Using compost, supercompost, or ultracompost reduces the risk of this happening, although the cost of the compost may outweight the benefits this involves - this is certainly the case for the rather expensive ultracompost, but might not be for the cheaper compost or supercompost.
To help you keep track of when your marigolds have finished growing, you may want to use an out-of-game timer and/or wear an amulet of farming.
When you expect the marigolds to be fully grown (roughly 20 minutes after planting), head to each of the patches you used and pick the marigolds. If they died, clear the patch instead. If they are alive but diseased, the nearby farmers sell plant cure for 25 coins if you talk to them - you won't get any marigolds this time around, but it saves you a seed. Once you have picked the flowers or cleared the patch, plant another marigold seed. You may want to add compost or supercompost before planting the seed. Then head to the next patch if there are any left, or a bank or the Grand Exchange when you're finished with all the patches.
Buying marigold seeds from the Draynor Seed Market
might be cheaper than buying them off the Grand Exchange
. The seeds are cheap enough compared to the flowers that this should not make a large difference, however.