Why on earth would you grow carrots in your greenhouse? After all there’s seldom shortages in the shops, they’re usually on sale year round and are inexpensive. Well simply because carrots are crunchier and sweeter when home grown especially when pulled just before eating not stored for months.
Then of course there are so many varieties with colours not often found in shops such as yellow, red, white and purple. (Orange carrots were developed in the 17C when they rapidly replaced older strains, these had unappealingly turned grey when cooked.)
So why not just grow them outdoors? Because carrots are not the easiest crop in the vegetable bed, picky as to soil these can be erratic germinating. But worst of all carrots almost invariably fall prey to Carrot Root Fly. This pest tunnels around the roots effectively ruining them. Also when grown outdoors carrots can only be sown from March to July for lifting through summer and autumn. The rest of the year they come from store or are left in the ground and dug as required, so slowly their quality fades.
Growing under cover completely avoids their fly problem. The warmer conditions makes germination reliable, and extends their sowing window by months at either end. This means you can produce fresh or nearly freshly grown carrots almost the whole year round. These are especially valuable in spring when they become most expensive in the shops.
Carrots need deep containers to develop long succulent roots. I prefer 10-12L ‘buckets’ with Nantes and Amsterdam forcing varieties (though there’s also spherical carrots such as Rondo to grow like radishes in shallow trays). Sieve their potting compost so it’s uniform or the roots may fork. Sow very thinly broadcast over the surface, sift over a thin covering of compost and tamp down. Stand the container in a very shallow tray and keep this full so water soaks up. From then on water from underneath and add liquid feed regularly. Most importantly, thin so they have room to grow, leave an inch/2.5cm between them. You will be surprised how fast carrots bulk up under cover and you can sow new batches weekly throughout most of the year to keep up a near constant supply.
One other point; if you don’t tell folk they’ll not spot what you are growing as the verdant foliage is so attractive and usually goes totally unrecognised.