spike [ spahyk ] noun: an unbranched, elongated inflorescence on the main axis, similar to a raceme but having sessile or subsessile flowers or spikelets maturing from the bottom up (indeterminate or racemose); an ear of grain such as barley or wheat
On a spike, the individual flowers are attached to the main axis directly or nearly so; on a raceme, they are attached by pedicels. In both forms, the flowers on the bottom open first. However, spike is a term rather loosely used by many gardeners, gardening sites, and sometimes botanical authorities to describe inflorescences with similar form.
Inflorescences that are Spikes
Left to right: Celosia spicata, Verbena hastata, Carex plantaginea.
Two plants with inflorescences that meet the technical definition of spike are annual Celosia spicata (wheat celosia), whose epithet means spike, and native perennial Verbena hastata (blue vervain), whose epithet means spear-shaped. Both have sessile flowers on terminal spikes that bloom from the bottom up. The erect flower spikes of C. spicata resemble sheaves of wheat while V. hastata spikes are arranged in a candelabra-like panicle. Spikes are also characteristic of the Cyperaceae (sedge family). Flora of Virginia describes sedge flowers as “arranged on the axis of solitary or clustered spikes or spikelets (the ultimate flower cluster of grasses and sedges).”
Inflorescences that resemble Spikes
Left to right: Liatris spicata, Lobelia cardinalis, Persicaria virginiana.
Although native Liatris spicata (blazing star or gayfeather) shares the same epithet as wheat celosia, its inflorescence is technically not a spike. Though its sessile flowers are borne on the main axis, they open from the top down meaning that they are determinate or cymose rather than racemose. Although often referred to as spikes (as in our Tried and True Native Plant fact sheets and in the Flora of Virginia), various floras describe them as spiciform or spike-like arrangements.
Other inflorescences that can be mistaken for spikes are the racemes of natives Lobelia cardinalis (cardinal flower) and Persicaria virginiana (Virginia knotweed, jumpseed). Superficially, both resemble spikes, but cardinal flower’s tightly-packed racemes can obscure its small pedicels and Virginia knotweed’s pedicels hide behind protective sheaths.
Reference
Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas.