Q: I planted Sensation Mix cosmos in winter sowing jugs this year. The plants looked great, but after I transplanted the seedlings to the garden, they got munched on by deer. The plants survived, but now they are very tall, with a ton of foliage, and they are barely flowering. Did the deer eat my flowers before they could appear, or did I do something else wrong? The plants get a lot of sun.
A: Cosmos are usually a trouble-free addition to the garden. They’ve got few disease problems, they come in all kinds of sizes and colors, and they bloom profusely. Usually. If they’re not blooming, it might be because of weather, how they were cared for, when they were planted, the site they’re planted in, or the variety of cosmos you’re growing.
Some varieties of cosmos can take a long time to mature enough to bloom. Sensation Mix can take as long as 90 days. But if you sowed yours in jugs outdoors in March or April, I’d expect at least some sign of blooming by now.
I don’t think the deer are the culprits here. People often deliberately pinch back the central stems of young cosmos plants so that they’ll branch more. It sounds like the deer just did that task for you. This could help account for your lush foliage, but I suspect other causes.
One big reason cosmos would produce a lot of foliage and few flowers is the richness of the soil. Cosmos actually prefer a soil that’s lower in nutrients, so if you gave the plants a lot of fertilizer or even just planted them in a bed that you had enriched with compost, they may have reacted by growing tall and bushy but not giving you a lot of flowers.
Similarly, cosmos prefer drier soil, so if you’ve been diligently watering them, they may have decided not to bother to bloom. Cosmos thrive off a bit of neglect.
Cosmos also need full sun to bloom well. That means six to eight hours a day of sunshine. Eight is better than six. And cold, wet weather can delay blooming.
It’s also possible that your cosmos have just been waiting for the days to get shorter. Cosmos are sensitive to day length, and if they are planted outdoors in the spring, they may not bloom until late summer, when the days are shorter than 14 hours long.
You can thwart this natural tendency by starting the seeds indoors, under lights. Hang the lights so that you can keep them within a couple inches of the seedlings, raising them as the plants grow. You can mimic short-day conditions by keeping your lights on for less than 12 hours a day.
There’s research showing that if you do this for a couple weeks before setting the plants out in the garden, you can get the plants to bloom earlier.
Remember that you need to treat seedlings differently if you grow them indoors than you would if you use the winter sowing method you used this year.
Seedlings grown indoors have to be hardened off before they can be transplanted into the garden. That means gradually exposing them to sunlight and wind, starting with an hour or two and then increasing the time they’re outside each day for about a week.
Hardening off allows the plants to develop a tough “cuticle” that protects them from sun and wind.
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