A short flue will usually give problems, but being outside isn't quite as bad as inside. Here's a graph showing the correct height of flues.
It's obviously not possible to build one that tall in the garden.
agree with what`s said - but chimney pots are sought after for garden planters Unless you find an unpopular one - Google " Marcone pot "and you`ll see one which is designed to increase draw - then find one on eBay - I did for £1
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