The unique challenge to carrot-growing in Houston is simply a question of depth. A productive local garden can be made with a mere four or five inches of raised-bed soil, but carrots grow to be much longer, or should I say, deeper than that. A few inches underneath the garden is Gulf Coast hard clay, and as my neighbor Tom noted, "You might as well plant them in concrete."
The first possible solution is to go the other direction, by building a length of mound, or planting two parallel two-by-six boards on their edges and filling up the space with garden soil. Another solution is to buy a variety of carrots that are stubby, like Nelson or Sweetness. A third way is to plant them in a large tub or planter.
Carrot seeds need light to germinate, so they can simply be scratched into the soil and watered lightly, or perhaps misted extensively until seedlings appear. Carrot seeds also require patience. Most Texas crops will sprout in a week or ten days, but carrots need two or three weeks before any pop-up action. After they do, each carrot only needs an inch or two of space to grow, so they can be thinned accordingly. After the sprouts start growing, carrots require a weekly fertilization.
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The final challenge for carrots is soap. As in, "Why do carrots sometimes taste like soap?" The reason is carrots produce two major ingredients: sugar, which is why giant bags of carrots are sold in Whole Foods for carrot juice, and terpenoids, a volatile compound that gives carrots flavor, and in high doses, makes them bitter and soapy. Unfortunately, there are many vagaries of soil, temperature, weather, and storage that affect terpenoids, so it's hard to control for supermarket produce.
One name for avoiding soapy carrots is Nantes, a French creation that provides more sugar and less terpenoids. The Scarlet Nantes variety is tasty, sweet, and short, probably the best choice for a Houston garden, and ultimately a delectable carrot soup.