Trees and shrubs can be inhibited by grass and weed!
There are many reasons why woody plants fail to make healthy new growth and sometimes die! Causes are from the common place such as planting too deeply, failing to water or watering too often to the totally bizarre, such as strangulation and urination. Today I have only one focus: competition from grass.
Grassing up to the base of young trees stifles their growth.
The stunting of tree growth can be quite extreme and sometimes a woody plant fails to grow at all and may be more susceptible to pest and disease. Not always, some very vigorous plants such as the Norway maple might not miss a stride.
|This vigorous Cedrus atlantica glauca tolerates the poor winter aeration in my deep wet soil and thrives|
As a tree eventually achieves some stature it starts to outgrow inhibiting grass.
In traditional fruit growing practice, reduction of vigour and promotion of fruiting was achieved by grassing down orchards. Such an effect might be an advantage to gardeners when in later years a tree threatens to outgrow its position.
|My Acer griseum has made very little growth in twelve years|
I often see inhibition of tree and shrub growth as a result of grass competition and have experienced it myself. Numerous trials by the landscaping industry have demonstrated dramatic effects. After several years in soil clear of vegetation a young tree might achieve double or treble the height of its grassed down neighbour.
Sometimes gardeners plant shrubs in a lawn and they often die for lack of summer water.
To my amazement I have witnessed arboreta that don’t understand this phenomenon. I have seen young trees just standing still for years in coarse weed or grass. I think that their green credentials do not permit them to get out the knapsack and underspray.
|Although grassed to the base I sometimes take out just a little grass to facilitate mowing|
It’s all about competition
Horticultural students are taught weeds compete for light, water and nutrients. We can discount light unless very small saplings are planted in very rough grass. Shortage of tree nutrients is significant but it is lack of water that makes the real difference.
Many gardeners are surprised how most tree roots grow in the top 30cm. That is the most fertile place in respect of air and availability of nutrients. A significant proportion of rainfall comes in small doses and just wets the surface. Shallow roots get first bite of the cherry.
Grass is a past master at making strong surface roots. It has evolved that way to survive on savannah. It is a moot point how the deeper roots made by un-mown taller grass compare with the dense matted roots of mown grass. Mown grass is usually the worst competitor because constant mowing keeps it constantly green and transpiring compared with tired old leaves on uncut plants. Dead grass might even give temporary benefit to the trees by mulching!
A particular problem of tall un-mown grass is at the base of a tree trunk when wet conditions prevail and wet leaves and humidity facilitates fungal infection.
I had intended to speculate about allelopathy where one plant chemically inhibits another. I knew that couch grass produces organic toxins that act as natural herbicides. I wondered whether such an effect might occur with other grasses. I found this site which claims allelopathy has a huge inhibiting effect on trees. Not all the allelopathic grasses quoted are relevant to UK lawns!
Kentucky bluegrass, red fescue, perennial ryegrass, Bermuda, and bahia grass all produce toxins. As much as 50% inhibition has been suggested. I have no idea how such a figure is derived!
Another danger is when closely grassed trees are damaged by mowers.
So where do trees grow best?
Trials have shown that they establish best in soils clear of vegetation. After planting such soil is best left undisturbed and kept weed free by shallow hoeing, hand weeding or spraying. The very best results of all are when weed free soil has a thick mulch. Most research into this has used materials such as bark mulch. If appropriate, don’t overlook loose gravel or stone.
In real gardens you will want to underplant the trees with plants!
|This tuberous corydalis is only above ground for ten weeks in winter and provides zero competition.|
|Winter bulbs generally thrive under deciduous trees|
So what can a gardener do to grow trees in the lawn?
Short of avoiding grass altogether to plant in clear soil circles is the usual solution. A meter or so diameter, although not perfect, will make a huge difference for the first few years. There will come a time that you can dispense with the circles. (and any ugly stakes - but that is another story).
I hate seeing trees planted in excavated depressions with deep edges that catch all the litter. Worse, if winter drainage is poor the sunken soil catches the rain. Best for your plants to be level with the lawn or in wet gardens slightly raised.
Personally if I want to plant a tree in grass I kill out the circles with glyphosate and plant direct into the lovely fibre-rich soil. I cringe when I see gardeners moving good turf - their very best soil - to plant trees!
What about ornamental borders under trees?
It’s probably not a good idea to grow grass under a grove of trees. You will get a load of green moss as a result of their shade!
Most of we gardeners will wish to grow mixed plantings of ornamental plants. To varying degrees such plants will compete for the trees’ water but dehydration will be much less than that caused by a lawn.
For most of us as trees become well established our new worry will be that the trees will dehydrate our plants!
In terms of a beautiful garden most of us will opt for tree plantings with a ‘natural’ looking understory of nice plants.
|Most of my own trees are planted in mixed borders|
A couple of caveats
Wise gardening words are often redundant in unusual circumstances. My own garden is a deep coarse silt - I have previously wrongly described it as fine sand - overlying sticky clay about two metres down. My perched water table ensures deep roots can always find water. My own trees are grassed up to their bases. It was a surprise how much the grass suppressed my Acer griseum and ginkgo but all my other trees have grown very well.
My advice to plant trees in clean soil applies to climates like here in the UK where dry periods in summer are rather frequent. My advice will not apply in places with high all year round rainfall. I saw a shrub border in Eire and another in Costa Rica where the complete planting was in mown grass and grew very well.
A tree planting scheme remembered
The promotion logo was …
‘Plant a tree in 73’
The next year it became…
‘Plant some more in 74’
A wag added…
‘None alive in 75’
The cynic had noted that the public were encouraged to plant trees in the countryside without any indication that without after care their chances of survival were small.
I saw many trees after planting invaded by coarse grass, ground elder, bindweed and all manner of dehydrating vegetation. No wonder many died or at best stood still for many years.
Bolton Percy Parish Council were informed that they would receive fifteen trees the following week. A planting committee was immediately convened. As the only horticulturist I was made chairman. In practice the committee was never quorate and I chose the sites and did the planting and all subsequent maintenance.
The trees that arrived were a curious selection. No wonder the nursery industry loved the scheme, they could sell all their surplus. It was proclaimed that each tree would have a ‘tree preservation order’. How stupid - any administration and preparation of plans would take a much greater time than planting and maintenance. I was not only too lazy I did not want to saddle people with future trees that might be in the wrong place, outgrow their space or even prove to be eyesores when perhaps not very healthy. The only plan of the whereabouts of these trees is in my head!
It is very difficult to suddenly find places for extra trees in a well wooded country village. My choices were rather desperate. It was the year before I started to maintain the then rather wild Bolton Percy Cemetery. Five trees were planted there. Twenty years later I was rather relieved when three by then large Sorbus aria, were chopped down when part of the old churchyard wall needed rebuilding. Every time I make my monthly maintenance visit I curse myself for the deep shade under a wretched huge sycamore I planted. On the other hand a beautiful Amelanchier canadensis in the parish room yard has thrived and produced beautiful blossom and autumn colour for the past forty years.
Last month I took it into my head to see how many trees have survived. I was pleased to find ten including a fine copper beech that will grace the village for the next hundred years.
|Planted in 1973 this copper beech is starting to make a fine tree. If you look carefully through it you will see another tree planted at the same time across the road. Sadly it is being strangled by ivy|
|Now worthy of a tree preservation order - hint to Parish council|