Planting carrots in September for a fall harvest is an excellent strategy known as succession planting, which extends the garden season and yields a superior crop. The appeal of a late-season planting lies in the cooler temperatures of autumn, which naturally enhance the flavor of the root vegetable. This second wave of carrots often provides a sweeter, more tender result than the summer harvest.
How Climate Determines Late Planting Success
The feasibility of a September planting depends entirely on calculating the time remaining until your region’s first hard frost. Carrots require a specific number of days from sowing to reach a mature, harvestable size, which is referred to as the Days to Maturity (DTM). To determine your planting deadline, you must count backward from your area’s average first frost date, adding the variety’s DTM plus a buffer of about two to three weeks to account for slower growth in shorter daylight hours.
A light frost occurs when temperatures drop to around 34°F, which carrots can tolerate, but a hard frost, typically below 28°F, can damage the foliage and compromise the root. For most standard varieties that mature in 70 days, planting should ideally occur 90 to 100 days before the anticipated hard freeze. Gardeners in northern regions must select a very fast-maturing variety and plant early in September, or even late August, to ensure the root has time to bulk up.
Selecting Varieties for a Fall Harvest
The primary consideration for a late planting is choosing a variety with a short Days to Maturity (DTM). Since the growing window is compressed, selecting cultivars known for quick development is crucial for success. Varieties from the Nantes group, such as ‘Napoli’ or ‘Early Nantes,’ are highly recommended because they mature quickly, often in 50 to 65 days, and produce cylindrical, blunt-ended roots.
These fast-maturing types are better suited for the fall than long-season storage carrots, like Danvers or Autumn King, which require 90 to 120 days. The small, spherical Paris Market type is another good option, maturing in as little as 50 days and ideal for shallow or heavy soils.
Sowing and Protecting Late-Season Seedlings
Carrots must be direct-seeded into deep, loose soil free of stones and clumps, as any obstruction can cause the roots to fork or become malformed. The ideal soil temperature for germination is around 72°F, often achievable in early September. Carrot seeds are notoriously slow to sprout, sometimes taking up to three weeks, and must be kept consistently moist during this period.
To manage potentially warmer September soil, a thin layer of fine mulch or a board placed over the seedbed can help retain moisture and keep the soil surface cool. Once the seedlings emerge and reach about two inches tall, thinning them to a final spacing of two to three inches apart is necessary for proper root expansion.
Snip the unwanted seedlings at the soil line with scissors rather than pulling them, which can disturb the delicate roots of the remaining plants. As the season progresses and temperatures dip below 40°F, young foliage can be protected from early cold snaps with a lightweight floating row cover.
Timing the Harvest
The fall harvest offers a distinct advantage in flavor due to a natural process called cold-sweetening. As ambient temperatures drop below 41°F, the carrot plant triggers an enzyme that converts stored starches into simple sugars like glucose and fructose. This sugar acts as a natural antifreeze, lowering the freezing point of the water within the root cells to protect the plant from damage.
This increase in sugar content gives the fall-harvested carrot its renowned sweetness, which can be further amplified by a few light frosts. Fall carrots should be harvested once they reach the desired size, which can be checked by brushing away the soil to examine the shoulder of the root.
In regions with mild winters, carrots can be left in the ground and covered with a thick layer of heavy mulch, such as straw or shredded leaves, allowing for fresh harvesting well into the winter.