SEEDHEADS AND HOW THEY DISPERSE

Last week I visited the Christchurch Botanic Gardens just on sunrise and headed toward the long herbaceous border. This awesome planting is my ongoing reference library of grasses, perennials, shrubs and my focus at this time of year, seedheads.

From a distance, I admired the border in the early autumn light (perhaps the best type of light of the year) and appreciated the muted, textural tapestry it presented to the early commuting walkers. I inched my way up each side with my camera, marvelling at the variety of crispy seed head forms as I went.

Words and photography by Julia Atkinson-Dunn

Developed in the 1920’s the herbaceous border at the Christchurch Botanic Gardens is thought to be the longest of its kind in public planting in the Southern Hemisphere.

Seedheads hold a strong visual attraction for me, and I have started to wonder more about their architecture which ranges so strongly between plant varieties. During an earlier investigation on pollination, I learned that flowers were designed to appeal to their preferred pollinators, with their shape, colour and fragrance marketing to a specific insect or bird customer. It hadn’t occurred to me that their resulting seedheads were also deeply designed for their preferred method of seed dispersion, yet another part of the process ensuring their species may live on!

On further digging, I found there are at least six prime methods of seed dispersion employed by plants, with many less-known ones added to note. The methods of wind, water and animal/human seed dispersal are described as allochory, meaning a ‘secondary agent’ is employed. Seed dispersal via gravity and ballistic methods is called autochory, referring to a plant’s ability to deploy seed on its own. Volume is the key to success and many plants hope to take advantage of as many seed dispersal opportunities as they can, often employing multiple methods, with some also using their roots and runners to spread as well.

Reviewing my photographs of that morning I compared the enormous fluffy heads of a giant cardoon to the gossamer of the grasses. Dense dark domes mingled with silvery grey spires and the fleshier, berry-like casings of others. Getting up close and personal revealed a world of structural interest as varied as a mixed bed in full bloom.

Guessing but believe these are cardoon seedheads.

Layers of seedhead interest in the long border at the Christchurch Botanic Gardens.

TYPES OF SEED DISPERSAL

Wind

Perhaps the most recognisable wind-dispersed seedheads are the wafting globes of thistle and the parachutes of ripe dandelions. We made wishes on these ‘fairies’ as children and watch them with a wary eye as gardeners. The helicoptering seed pods of sycamore trees and acers flutter to the ground, weighted by their seed and buoyed by the sail of their very fine casing. This is with the hope they will be carried further than directly below the trees’ shaded canopy.
Many of us would have also noticed the rolling spikey seedheads of our coastal dune grass Spinifex sericeus peeling off down the beach during a windy day!

Water

One of the most cleverly designed seeds destined for water dispersal would have to be the coconut. Its buoyant, water-tight casing is packed with plenty of nutrients for any embryo to last months at sea, making it one of the most widely spread plants on the planet. Have you ever noticed kowhai trees in the wild situated near streams and rivers? Their tough, linear seed pods allow them to bob downstream before eventually finding a successful spot on land to germinate.

Hitching rides

As children with hairy legs bushwhacking in the Marlborough Sounds, we’d always return to torturously pluck off the hooked seedheads of Carex uncinata. I laughed when I discovered its common name of ‘bastard grass’! Like this native hook sedge, many other types of grass rely on catching a grip on humans and animals to disperse themselves more widely, unfortunately, many of these are also invasive which is why we should check out clothes for rogue hitchhikers.

Thinking this might be the seedheads of *Anise hyssop*.

Digested

Seventy per cent of our native plants have fleshy fruit or berries that are considered a welcome food source by birds and thus pooed out around the country. Not all are what we as humans would consider delicious. The enzymes in a bird’s digestive system break down the waxy protective coating so that when ‘dropped out’ they are ready to engage with soil.

Echinacea and sanguisorba seedheads in my garden which are regularly feasted on by birds.

Gravity

The simplest form of seed dispersal is employed by so many of our garden plants. On closer inspection you will see many of the little seeds that cluster together on a head are spear-shaped, some even with a little fin or feather on the end that help them to aim downward, and with some luck, penetrate the soil for success. These seed heads are often an appealing snack for birds too.

An echinacea seed head in my own garden, partially dispersed. You can see the spear shaped seeds which allow it to fall vertically and hopefully pierce the ground for a better chance at germination.

Ballistic

This is such an aggressive term to give to a natural process but on experiencing the long thin pods of my arch nemesis weed Hairy Bittercress (Cardamine hirsute) audibly and visually explode with one light touch - it truly seems an accurate description. Other weedy mates like gorse and broom use the same system for propelling their seed, and by also being very prolific in their flower production, they have great success at spreading. Closer to home in the garden, pod-based plants like peas, poppies and innocent-looking impatiens crackle and burst their pods as they dry.

My niece opening the mature nigella seed head - known to pop and spread it’s seeds.

Other fascinating methods employed by plants include seed dispersal by fish like the tucum palm and its partner, the pacu fish in the freshwater wetlands of Brazil. Certain pines, eucalyptus and banksia have cones that are so firmly sealed with resin that they only open and release their seeds under the heat of fire! And even the humble acorn is designed to be burrowed away in caches by squirrels where it finds a lovely spot to germinate if the creature forgets where it is.

This is an expanded version of the article featured in my Stuff ‘Homed’ gardening column for beginners, The Press, Dominion Post and other regional papers on April 27th 2023.
All words and images are my own, taken in my home and garden in Christchurch, New Zealand unless otherwise captioned.