Cosmos - how high?!

Can anyone help me with this...

I have grown some cosmos 'sea shells' ( Johnsons world botanics) and they are now turning from big seedlings into little plants. I had ordinary cosmos last year and loved it before I had re-planned my garden, manured and mulched.

Having looked up cosmos a little, because I am growing from seed wanted a bit more information, I have now discovered that cosmos are from Mexico and like hot, poor soil!
The cosmos I had last year only grew to about a foot and a half, and they did not spread much at all - infact I left the suggested spacing and there were gaps between them all. I did get from the garden centre though as small plants. But I have since read that they need a spacing of 45cm (!!!) and grow to somewhere between one and one and a half meters!

I have spent ages getting the soil in better shape, and have mulched with forest bark to keep it moist and help introduce organic matter.

The beds I was planning to put it into are mounded, so there is good drainage, just that it is all nice and moist underneath.

1. Will my cosmos hate my nice soil?

2. Are they likely to grow into enormous triphids which look ridiculous being two feet taller than all my highest plants?

Does anyone have experience of cosmos and any advice? I'd be very grateful.

PS I have a bed at the back of the garden, un-mulched and dry, but it only gets sun until midday. Would that be enough sun for them, and would they prefer it there?!

 

Hi Victoria.

There are different varieties of Cosmos. Some types will only grow to about 2 feet and some are more like 4 feet. Personally I always like the larger ones - they are more impressive. The seed packet should indicate the final height.

Another point is that Cosmos, like Cleome and Tithonia are tender annuals. Christopher Lloyd (Great Dixter) used to suggest sowing ordinary seeds on 1st March, but not sowing these three till 1st May. The reason being that if they are put out too early and are hit by a period of cold weather (even though it may be above freezing) they have a tendancy to sulk and never properly recover. I have personally experienced this. This could explain a poor showing.

I think they also like the heat. One year I had magnificent Cosmos, growing uo to 5 feet high and nearly 5 feet wide, and flowering into late Novevember, in rich well fertilised and well water soil. But that was a hot year, since then they have been good, but never that good again. I suspect that they could do well in the bed at the back of your garden. But like any plant that you hear likes poor, dry soil - they probably like rich well watered soil even more. If the soil is too rich they risk producing too much green growth at the expense of the flowers. But if you have lots of flowers, you don't have a problem.

 

Thanks Peter,

The packet states that it needs 45cm spacing and grows to 1m to 1.5m - Eek! I think I will be putting it in the back bed then, and perhaps in the side bed - not mulched at present and a bit more sheltered. It will get sun there from midday onwards. I guess I just wondered if it the packet was being on the optimistic side!!!

I just can't quite imagine the little plants that I have growing into monsters - it doesn't look possible!