The Herald - The Herald Magazine
Gardening with Dave Allan
WEEDS challenge us all. Take your eye off the ball and they’ll smother the garden, throttle your plants and seed far too generously.
Understanding what makes a plant, even a weed, tick is the best way of assessing whether your usual gardening methods work best or if they need to be modified.
Weeds need the right conditions to germinate: moisture, heat and light. And timing is critical as instant death is the reward for getting it wrong.
Many plants, like knotgrass, need warm spring weather after winter chilling and without the right trigger by May, the seed stays dormant till the following year.
In the veg patch or ornamental beds, soil between plants is either bare or mulched, so I’ll see how each of them affects our war with weeds.
Moisture is pivotal, with seeds germinating immediately after rains. Our worst foes are slow compared to the South African klapperbossies which start seconds after contact with moisture and sprout after six hours.
I fondly remember that blissful dry weather in spring without a weed in sight. Then came the rain and a fearful green flush.
Good old shepherd’s purse, annual meadow grass and groundsel were all back, but only on bare soil. The mulched areas remained pristine.
And chickweed. It’s a real menace. Different generations flourish all year round, with each plant producing 250 seed cases containing up to 10 seeds. In winter, its opposite leaves have been seen folding together to protect new growth.
Perhaps worst of all, chickweed can carry viruses like mosaic virus, the scourge of courgettes.
Frequently hoeing bare soil to catch ‘em young is best, but almost impossible when it’s wet. Limit this disturbance to the top 2 or 3cm as we don’t want to disturb the millions of seeds of every age and size in our garden. Darwin seemingly identified 537 seeds in three tablespoons of pond mud.
Most seeds deeper than a few centimetres don’t get enough light to germinate till we come along with our spades.
Every time we cultivate the ground we bring fresh seed near the surface, so no dig saves a lot of trouble.
Seeds can identify and battle with neighbours in their struggle to germinate. They use light receptors to assess if the soil above is vacant.
They distinguish between the more intense red light waves directly hitting the ground and light which is weaker after passing through leaves that absorb some light in photosynthesis.
A seed is unlikely to germinate immediately beneath an established rival.
So weed germination is severely hampered beneath green manures as well as broad-leaved plants. And the same applies for any seeds lurking beneath a mulch.
Seeds also monitor fluctuations in temperature as this is another sign of bare ground above. But the temperature beneath a mulch varies very little, so again, there’s no trigger to get germinating.
But some weeds outwit us all. I was only completely converted to no-dig a few years ago so am still battling with perennials like couch and ground elder.
They keep peeking up along the edge of beds. I’m sure that by forking out what I can easily reach, I’ll finally beat them, remembering my triumph years ago in a war with couch in a corner of the polytunnel.
Deep-rooted docks are always a nightmare and must be dug out before seeding.
With up to 60,000 seeds that could remain viable for 70 years, get them young.
No-dig and mulching are best for weed control.
But when you remove mulches for roots like carrots or any ornamentals needing free drainage over winter, opportunistic weeds will have the last laugh. Till they end up in the compost heap, of course.