Carrots have a strict growing routine, regardless of when you sow them, so accepting that there is no strict “best time to thin carrots” is crucial to your success. Carrots don’t listen to dates and have one of the longest season ranges of any garden vegetable. 

You can sow carrots as early as February, or as late as August, so the only way to determine the best time to thin carrots is by looking at their top growth. Unlike most other veg, it’s best to thin carrots twice, or as often as you need.

The more you thin them, the bigger your final crop, and the more often you’ll have baby carrots for dinner. Don’t see thinning carrots as a garden chore, see it as a chance to make an early harvest.

When to thin carrots

Deciding when to thin carrots should be done by size, not date, as you can sow them from early spring right through to late summer, usually thinning as soon as they’ve germinated, and again 2-3 weeks after that.

Thinning seedlings should be second nature to any gardener. Unless you’re growing micro-greens, every crop in your garden needs space to grow to maturity without competition, and because carrots should be sown directly outdoors, you don’t have the benefit of spacing them out as you do with indoor sown plug plants.

Once you have thinned out the seedlings, you will need to thin them again when the tops are around 10cm tall. This will give you a decent crop of baby carrots halfway through the growing season, and give a brilliant boost to carrots left in the ground. They’ll thank you for getting rid of their competition.

First thinning

Thinning carrots is one of the simplest processes in the veg garden, always thinning based on the height of the top growth, to the same spacing as the top growth. When your carrot seedlings are around 1-2 inches, tall, thin them to 1-2 inches apart.

Water the soil well before trying to thin carrots as they will come away from the soil with less resistance. Be gentle, and try not to break the stems as this will encourage carrot fly (more on that later).

If you can, avoid disturbing the soil as you lift the tiny carrots out by gently pulling from the base of the leaves. That is best practice, but if your soil is too heavy, or the root just won’t budge, try teasing it with a cocktail stick, or the end of the pencil, trying not to damage any of the roots you intend to leave in place.

  • Tip: Carrot seedlings are one of the best microgreens you can grow, and absolutely packed with flavour, so rather than chucking them on the compost heap, scatter them over a salad. The roots will be sweet and crunchy, and the tiny tops will add a distinct fragrance to your meal.

Second thinning – and early harvest

Around a 1 month after thinning out, your carrots will need a little extra help. You’ve thinned the carrots as seedlings, but now they need at least 10cm (4in) each to grow to their full potential.

As soon as your carrot tops are 4 inches above the soil, they need thinning again. But fear not! These mid-season carrot thinnings are not going to waste. You can transplant them (more on that later), or better yet, eat them. They might not be huge, but baby carrots are sweeter, more tender, and frankly, a more exciting crop than their fully-grown rivals. 

Don’t waste anything, the leaves are perfect in salads or stir-fried with spinach.

  • Tip: After the second thinning, top the soil surface off with a little extra compost to keep the light away from the top of the carrots as they mature. Any root above the soil will discolour, and either become tough, or fall victim to hungry insect life. Last year we had a nightmare with sparrows pecking at bugs around exposed carrot tops.

Transplanting thinned carrots

There are two reasons to transplant thinned carrots, and it depends on when you’re doing it.

For your second thinning, your carrots should be strong enough to withstand moving. If you move them gently to a new location, they will continue to grow and you will get shorter, stunted, carrots, but I’ve found they tend to have much sweeter roots. There’s no science behind it, I guess it’s something to do with the stress, and forcing the plant to do more work underground to re-establish. 

Or if you don’t feel like carrots for dinner, then transplanting carrot seedlings can be a great way to get budget blooms next year. Carrots have biennial cycles, so you get tasty roots one year, and the next year the toughened roots send up beautiful umbellifers. Plant direct into your borders at any time of year, and they will flower the next year. Plant them around 10-15cm apart, direct into the soil. You’ll forget they’re there until next year when you’ll have a beautiful frothy addition to the garden and cut flower beds.

Avoiding Carrot Fly while thinning carrots

Carrot fly are attracted to carrots at all times of year. The carrot growing season is early spring to late autumn, and you should be thinning out at least twice during that time, usually in the hot dry months when carrot fly is at its worst.

Carrot fly are attracted to the smell of carrots, which is released every time the leaves are touched, so choosing the best time to thin carrots isn’t just about finding the right day, it’s about finding the right time. Early mornings, when the air is more humid will dampen the smell, or better yet, thin carrots on a rainy, windy, day to confuse them even more.

If you have it, netting or garden fleece can be placed over your carrots for a few days after thinning so you have a physical barrier between the irresistible smell of damaged top growth and the villainous carrot flies.

If you choose to transplant thinned carrots, it is best to add them to pot displays. Carrot flies fly low to the ground and are much less likely to find the scent trail of bruised carrot stems if they are raised above ground level. But any extra precautions you can take are worth it – they are determined little fiends, and will try their very best to find roots to lay eggs on.

  • Tip: My tip for avoiding carrot fly? Don’t risk putting any thinned carrots on the compost. It risks carrot fly in your compost heap, and if there’s one thing you want to avoid, it’s using infested compost on your crops next year – we’ve had it before, and you won’t realise until it’s too late.

Conclusion

So regardless of when you sowed your delicious colourful crop, remember that the only thing carrots ask for is observation. Remember those two main signs: Thin seedlings at 2 inches tall, to 2 inches apart. Thin mid-season carrots at 4 inches tall, to 4 inches apart.

If you’ve got that right, you can’t really go wrong, but take precautions against carrot fly, and make sure you don’t let your tiny mid-season harvests go to waste.

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