BrightKidz Library
Subjects
Seedlings crowded under a parent plant, and one growing in the open beyond its reach A grown plant stands on the left. Its leaves are drawn as a row of separate oval shapes arching over the ground, and a straight stem holds them up. The ground directly beneath those leaves is shaded, and a dashed upright line marks the far edge of that shaded strip, where the outermost leaves end. Inside the shaded strip three tiny seedlings are packed close together, all of them short and stunted. Well beyond the dashed line, out on the open unshaded ground, a single seedling stands on its own and has grown several times taller, with two full leaves of its own.

Discovering the Secrets of Seed Dispersal

About 18 minutes

You already know how a seed travels — on the wind, on the water, hooked into fur, or fired out of a pod. What none of that explains is why a plant would go to the expense.

Because it is an expense. A parachute has to be built. So does a hook, and a husk thick enough to cross an ocean, and a pod wound tight enough to fire. Every one of those is made of material the plant could have spent on leaves or roots instead. A plant that skipped the whole business would have more left over.

So something must make it worth paying for.

The competitor is the parent

Here is the part that catches most people out. The plant most likely to crowd out a new seedling is not a rival species, and not the plant next door. It is the seedling's own parent.

The parent is not being cruel. It is simply already there, and already bigger. Its leaves are spread above the exact patch of ground its seeds drop onto, so it gets the light first. Its roots are already through that soil, so it gets the water first. A seed that lands underneath is starting a race against something that has been running for years.

Three things go wrong underneath

Light arrives second. Whatever the canopy above does not use is what reaches the ground, and a seedling has to live on the leftovers.

Water arrives second. Established roots are thicker, deeper and already in place. They drink first.

And the ground underneath is the most dangerous ground there is. This is the one people find surprising. Insects and fungi that specialise in one kind of plant gather around a grown one, because that is exactly where their food is. So the soil directly below a parent is crawling with the specific pests that attack that specific plant. A seed that gets clear of the parent is not only escaping the shade. It is escaping the parent's pests.

The edge is a real line

All of which means "far enough away" is not a vague idea. It is a distance, and you can go outside and measure it.

A grown plant reaches out over a circle of ground — out to its furthest leaves. Inside that circle it is taking the light and drinking the water. Outside it, the ground is open. Gardeners have a name for the edge: the drip line, because rain running off the outermost leaves lands along it.

Every seed the plant makes is trying to get past that line.

Watch what stays still

Drag the slider to move the seed further from the trunk. Then take your eyes off the seed and watch the drip line instead.

A seed moving out from its parent plant, past the fixed edge of the parent's reach A grown parent plant stands on the left, its leaves drawn as a row of separate ovals arching over the ground on a straight stem. The ground beneath those leaves is shaded, and an upright dashed line marks the far edge of that shaded strip, at the point where the outermost leaves end. Gardeners call that edge the drip line. To the right of it the ground is open and unshaded. A single seed rests on the ground with a crown of fine threads above it, like a dandelion seed, and it slides further and further to the right as the slider is moved, carrying a label showing how many metres it is from the trunk. The dashed drip line stays in exactly the same place the whole time, and the shaded strip never changes size. the parent drip line open ground 1 m The drip line never moves. Only the seed does.

Move the seed out from the parent and watch where the drip line sits.

The drip line never moves. It does not shrink as the seed travels, and it does not follow. It is the same line at twelve metres as it is at one, because it belongs to the parent and has nothing to do with the seed at all.

That is the whole thing in one picture. The seed is not making the problem smaller. It is getting out from under a problem that stays exactly the same size — which is why distance is the only answer available, and why a plant will pay for a parachute to get it.

Measure it for yourself

Now find out how far your own seeds can actually get, and whether it is far enough to clear a plant.

Testing how far three seeds can be blown An experiment set out along a measuring track. On the left an electric fan stands on a base behind its round guard, three blades showing. From it a long strip of card runs away to the right, ruled all the way along with tick marks like a ruler. Three dotted flight paths of different lengths leave the fan: the shortest drops a pebble almost straight away, a longer one carries a bean seed to the middle, and the longest carries a feathery dandelion seed right to the far end of the track.
Same fan, same starting line, three different seeds. The distance each one reaches is the thing to write down.
  1. Set up. Put the cardboard on the floor and place the fan at one end of it.
  2. Wind test. Put a dandelion seed on the cardboard, switch the fan on, and measure how far the seed travels.
  3. Compare. Do exactly the same with a bean seed, and then with the pebble. Same fan, same starting spot.
  4. Water test. Drop each kind of seed into the bowl of water. Which float, and which sink straight away?
  5. Write it down. Record how far each seed went, and which ones floated.
  6. Now measure a plant. Go outside and find the plant that seed came from, or any plant of about that size. Measure from its stem out to its furthest leaves. That is its drip line.
  7. Compare the two numbers. Did your seed travel further than that plant's drip line, or not?

What your numbers mean

Step 7 is the one that matters. A seed that travelled further than the drip line has escaped. A seed that did not is still under its parent, competing.

A dandelion has a small rosette of leaves and a seed that flies, so its seeds clear their own drip line easily — which is why dandelions turn up all over a garden and not in one clump. Your bean seed almost certainly did not clear anything. That is not a failed seed. It is a seed with a different plan: heavy seeds are the ones that hire an animal, and an animal carries them much further than any fan.

There is no single best way to travel. There is only far enough for the size of the plant you came from.

Why is the ground directly under a parent plant such a bad place for its own seedling?

Next time you stand under a big tree, look up and find the place where the leaves stop, then look down at the ground beneath that edge. Every seed that tree has ever made was trying to get past that line. The parachutes, the hooks, the husks and the fired pods are all the same answer to the same problem: the hardest plant to grow beside is the one that made you.