Answer: The most common reasons for short carrots are heavy soil, rocky soil, low fertility and crowding -- the best carrots grow in a sandy soil which is moist yet well drained, has lots of organic matter worked in and is deeply prepared. Hot weather and drought can also stunt carrots. Carrots are usually planted in soil which has already been prepared based on soil test results with a pH of between 6 and 6.8 and a reasonable level of fertility. Occasionally a side dressing is added in mid season if the tops seem to be in need of it; potash is in fact the most important for good carrots. All in all, though, it is really too late in the season for fertilizer to help.
Depending on the variety you planted, your carrots may have a general growth to harvest timing of anywhere from 60 to 85 days. Add to that about two weeks of "fall factor" time to compensate for slowing growth as the season winds down, and then count backwards from your frost date. That's when you should have planted your carrots in hopes of harvesting mature carrots this fall. (To see if your carrots are still growing, look at the tips. Immature tips are white, mature tips are carrot colored.) Keep in mind, too, that carrots can be eaten at any size and will hold in the ground until winter if mulched deeply. If you can protect the tops through the first frosts they'll continue to grow albeit slowly unless of course they are already mature.
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