Question
I started a marigold in a 4" container from seed, it was a yellow small flowering type with uniform sized petals.  As the plant grew it was transplanted into larger containers until it ended up in a plastic half barrel where it remained.  The marigold was watered daily and kept in full sun on the eastern side of the house to get morning and early noon sun, but protected from the burning south Texas sun during the summer.  Deadheading was performed daily to prevent seeds forcing a constant almost full coverage of blooms.  It was root bound and had to be watered in the morning and evening.  It was watered with regular tap water with a very diluted (1/10 strength) of Miracle Grow fertilizer added.  Unfortunately during a storm I didn't get outside to raise the protective screens around it and a gust of wind split the plant in two at the first set of branches.  I had the plant for almost 14 months when this occurred and it had developed a thick trunk complete with a type of bark like material on the trunk, it acutally looked more like a tree than an annual.  I found the small flowering marigolds to be easily adaptable to this technique but the larger flowering types tend to split their branches due to flower weight.  The plant reached almost 3 feet high and did suffer one major broken branch due to my carelessness in handling during the last transplant. The side with the broken branch was always facing away to I only saw the good side.  The dense green foliage in contrast to the vivid yellow flowers was breathtaking.    I suspect it would have kept growing for months until the weight of the leaves broke the branches at the trunk.  By the way you cannot get water on the leaves when the plant gets this large or the weight will easily snap the branches.  I am very interested in repeating this technique with another marigold and was wondering if there is a name for forcing annuals to live longer lives by not allowing them to seed.  I have been unable to find information about this gardening technique and was wondering if anyone else has tried it.  Any information regarding the maximum life of marigolds or to technique to make them last more than a year would be greatly appreciated.

Answer
I am so impressed with the size of your very green thumb, and your diligence, Kelly.  Your enthusiasm is so infectious I have been looking for answers everywhere I could think of to see if anyone has addresses this.

There are lots of different species of Marigolds, some perennial and others annual, some large and some petite.
'Tagetes erecta' is one very large, annual, African Marigold (originally from Mexico) with huge blossoms that grows up to 4 feet tall.  Crosses with the French species have produced many hybrids sold in Northeastern garden centers where I do gardening - I must admit I know little about the Texas gardening culture, and I should probably not be attempting ot answer your question ro help you with any guidance but as I said your enthusiasm is so infectious.  

Marigolds by the way are very popular cut flower plants in India.  

Park's Seed highlights the Antigua Hybrids of this plant for their longevity, way beyond the duration of bloom you can expect from other hybrids.

Auburn University's Cooperative Extension has posted a page specifically on Tagetes (www.ag.auburn.edu/hort/landscape/Marigold.htm) which I think you would find somewhat helpful.  Note that flowering in T. erecta is determined by daylength.  Supplemental lighting and even exposure to Carbon Dioxide are used to boost bloom size.  

Purdue University however has a website (www.hort.purdue.edu/newcrop/proceedings1996/v3-546.html) that details growth habits and culture of a total of 8 Tagetes species - half annual, half perennial.  Some marigolds you see will continue to flower until frost finally ends that story - theoretically at least, that could be indefinite.  If the plant however is building a wood stem, at some point the plant may simply not be able to support further growth.

The Journal of Natural Resources and Life Sciences Education (crop.scijournals.org/cgi/content/full/43/6/2118) posts detailed cultivation problesms and solutions on its Crop Sciecnce pages for Marigolds under one study, "Optimizing Marigold (Tagetes erecta L.) Petal and Pigment Yield".  Note their comparison of transplants and seed-grown Marigolds for vigor and flower size.  There is also a reference to Nitrogen deficiency in one plant that does not appear to be unusual but is easily remedied.

I wish I could guide you to more relevant research on this.  But no one I spoke with was familiar with any studies that take into consideration the life span of a Marigold, or that attempt to extend the life expectance beyond the pale.  You would be the first!  This is the best kind of research.

Very impressive - thank you for an interesting question.  If you need any clarification please write.