When I am writing I always try to be accurate and so describing ‘spikes’ of flowers is something I always try to avoid. My default is ‘spire’, mainly because it sounds similar and because my brain will have caught up with my fingers by the time I get to the fourth letter and managed to prevent me typing ‘k’ and substitute an ‘r’! ‘Spire’ also describes the sort of towering effect I mean. So why am I (usually) so careful to avoid the word ‘spike’? It is because, although it is clear, from using the word, that I mean a tall, slender collection of flowers, ‘spike’ has a very precise botanical meaning and most of the ‘spires’ in the garden are not spikes at all but ‘racemes’.
The difference? Well to be a spike, the stem has to be unbranched and the flowers, that open from the base up, are joined directly to the stem. It is this lack of individual flower stems (or pedicels) that is the key.
Most of the flowers in the garden that have ‘spires’ of flowers, including digitalis (foxgloves) top, foxtail lilies (eremurus) above and antirrhinums below are racemes. In many cases each pedicel has a small bract or scale at the base too. True spikes are rather uncommon but plantains (plantago) are an example.