How to make Cosmos Bloom?


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Hi, I'm only 20 and this is my first year gardening. I had planted Cosmos (sensation and bright lights) along with my vegetables in my garden back on May 10th. I used MiracleGro all purpose plant food a couple times over a few months. The plants have grown big with a lot of leaves but I see no evidence of flowers coming in; there are no stems growing with buds on them, and the estimated bloom time was July 15th. I learned adding too much nitrogen to the soil can make plants produce too many leaves but not enough flowers/fruit - And Cosmos are particularly susceptible to this.

Is there anything I can do to the soil to make them bloom - Such as try adding the MiracleGro Bloom Booster fertilizer (for more phosphorus)? Should I wait longer? Do Cosmos flowers take long to bloom after buds appear? My Zinnias had buds for almost two weeks before they bloomed.
 
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Hi @cirroalex i don't know what part of the US you are in, but more than anything Cosmos like sun and heat. I have never fertilized the cosmos i grow and i get more blooms than vegetation. With nitrogen, they will put their energy into making leaves and not blooms. Best to wait, ignore them for awhile, don't give them any more nutrients. If you have seeds, scatter them on poor soil and if you have a month of summer left they will grow and bloom.
 
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I have a few that seem reluctant to bloom too and what I have noticed is they are bigger. I don't recall variations in brand and variety of seed planted, or if there is a direct correlation there.... but ones that are smaller when mature bloomed early and fairly quick. Now there are some that are like 6' plus tall and those seem to be much slower to bloom as they keep getting bigger and taller. But I do have faith as one of the 6 footers has bloomed and it is magnificent.

Some of my Cosmo's are in containers and some are in the ground.

There is one set (of 4 Cosmo's) in a container that has caused me to wonder if it is going to bloom. So I goggled up and this seems accurate. I have some Tomato fertilizer so I might give it a try. Having as much variety of plants growing as I do I'm guilty of sometimes just giving most everything a little 10-10-10 ;)

So a quick google...
The cosmos might either be suffering from a lack of light or a surfeit of nitrogen, which is causing them to promote more leaf than flower. Tomato feed, which is usually applied to encourage good fruit formation and ripening, will promote your cosmos to flower if you have the same problem next year, but try them in the sunniest spot possible in soil that is free of any fertilizer and see what happens.

So as it relates to Tomato fertilizer.
Looking at the NPK for general Tomato fertilizer (say 2-5-3) it has reduced (2) Nitrogen (growth of leaves on the plant). Higher Phosphorus (5) (root growth and flower and fruit development) and moderate Potassium (3) overall functions of the plant.
 
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Agree with Beverly .....no feeding even with tomato food and keep on the dry side.
I always pinch back cosmos and do so several times to make strong, sturdy multi branching stems. Best left to grow without fussing them.
However, cosmos can be a fun plant.......
I often grow a few in pots and remove all flower buds until August when they are about 5' tall. They are then planted out, staked and allowed to flower. You can tie them against a wall or fence where they will make large flowering plants at least 6' tall.:)
 
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I'm in a Zone 6a so I still have plenty of summer left. I think instead of fertilizing them with a fertilizer with less nitrogen/more phosphorus, I'll leave them alone and water them less. Thanks for the advice everyone!
 
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That's weird, I was going to post about the same issue! I bought 8 cosmos seedlings and all but one are doing great (mostly in containers.) But the one right by my front door is 4-5' tall, but not a single bud or flower.

Mine get adequate to good sun, are fertilized, and I changed all my container soil this spring and added some organic compost too. Have grown cosmos for years and they are usually reliable flowering all season.

But now I have this one freakishly large one with zero flowers, it's like all the growing energy went into leaf production.
 
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I had 2 Bright Lights in 5 gallon cans that a friend gave me. One was 3 1/2 feet tall, in full beautiful bloom, orange flowers all over. The other one was 4’ tall, lush green leaves way denser than the other with 0 blooms. 3 months later the orange one was mostly bloomed out, looked tired, mostly seed heads. The full bushy one had 4-5 flowers, pretty yellow, lots of green leaves. That was December. In January the yellow one explodes into brilliant bloom, flowers everywhere, gorgeous. Now the orange one is gone and I cut the yellow one down to 15” and it still has flowers! Gotta love genes and weather.

So, be patient, it will happen when it happens.
 
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I had 2 Bright Lights in 5 gallon cans that a friend gave me. One was 3 1/2 feet tall, in full beautiful bloom, orange flowers all over. The other one was 4’ tall, lush green leaves way denser than the other with 0 blooms. 3 months later the orange one was mostly bloomed out, looked tired, mostly seed heads. The full bushy one had 4-5 flowers, pretty yellow, lots of green leaves. That was December. In January the yellow one explodes into brilliant bloom, flowers everywhere, gorgeous. Now the orange one is gone and I cut the yellow one down to 15” and it still has flowers! Gotta love genes and weather.

So, be patient, it will happen when it happens.
Will the Orange Cosmos have new flowers later this year?
 
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No, sadly it succumbed to a hard frost, but the yellow one was still going strong so that helped it survive. I did collect some seeds and I just planted them yesterday so there should be a new generation of giant yellow and orange cosmos in a few weeks.
 
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I hope that the Orange Cosmos will grow. Orange is a brilliant color.
 

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