Wednesday 23 October 2013

(7) Using glyphosate selectively; selectivity by timing

 As an annual the  poached egg plant will die in July. It will be an opportunity to spray and give self sown seeds a clean start
I make the usual provisos that glyphosate is not selective in itself. It does not distinguish between a plant and a weed. You make that decision when you spray. 
Selectivity by timing relies on your plant being dormant when you spray glyphosate on growing weeds.

Particularly relevant to my theme today is that glyphosate in normal circumstances only acts through the leaves and other green parts of the plant. It does not kill plants lying dormant below the ground and for normal practical purposes does not wash into the ground and leave residues that will damage future growth.

Glyphosate is translocated through the leaf to kill the root. It is useless against chopped up little pieces. It needs an intact growing weed and it is of no value to zap a weed like convolvulus when it has just emerged from the ground. Weeds need plenty green foliage to be killed. A well timed spray to an intact vigorous weed like couch or convolvulus in favourable circumstances will kill it in one go. It is more common however to have to repeat spray some weeks later.

This post is part of a series. I do not want to persuade inexperienced users to follow my methods Certainly not until they have read my previous posts. Insert ‘glyphosate’ into my search box to find out more.

Selectivity by timing
The obvious example of this is spring bulbs that die down in summer. Daffodils for example might be growing in a stand of luxuriant perennial weed. You can safely spray when the bulbs have died down. I have written before about the thousands of daffodils, snowdrops and bluebells which when released from the yoke of ground elder, couch, horseradish and nettles returned to their former glory in my churchyard gardens. The same principle applies to any bulb that goes dormant whenever in the year this might occur. Hardy cyclamen for example lose their leaves between March and  August. It is safe to spray then.

Selectivity by timing can be used on any garden plant that dies down and goes dormant. Many herbaceous perennials die down in late autumn. Even nasty weeds like couch if undisturbed when entangled at the base of a dormant perennial will succumb to glyphosate as late as an December application. You cannot do this amongst those herbaceous perennials that in autumn are already making new green basal growth. 
You can often further increase your window of opportunity with herbaceous perennials that are becoming senescent by prematurely cutting them down to the ground. This comment applies to where you might have a severe problem. I would never normally cut plants back early for routine control of weeds.

A slightly different aspect of selectivity by timing is to spray at the end of your crop. It is appropriate in the vegetable garden to spray to ‘clean up’ between crops or in the winter if the land is fallow. Slightly more subtle in the ornamental garden is where annual plants like limnanthes, nigella and Nemophila ‘Penny Black' naturally die and there is a window of opportunity of a few weeks to kill weeds before self sown seeds germinate. You can even anticipate this event by spraying a little early. 
For some of you, limnanthes may be the weed and you can spray it anytime! On a slightly different note where a plant from seed is as prolific as limnanthes it matters little if there is some collateral damage when you spray.

Nemophila ‘ Penny Black’ No weeds here, it had a clean start
This love-in-a-mist will have already self seeded. I am in no hurry to remove it as I like the dry seed heads. Nasty milkweed has germinated below. I can safely spray it.
Some of the examples I have given involve spraying in late Autumn or Winter. It surprises me how many gardeners do not take advantage of spraying green weeds at this time of year. Yes, the glyphosate is much slower to kill, but against new weeds from seed it is extremely effective at this time when the soil is wet and hoeing is useless. Where plants such as hosta, agapanthus and phlox and indeed most herbaceous plants completely die down you can spray with complete freedom all winter. Where perennial weeds are as  dormant as the plants however glyphosate will be of no value! Take care not to spray un-noticed small Spring sprouting shoots that  are about to appear.

Seasonal variability in plant’s sensitivity to glyphosate

  • There is considerable variation in the sensitivity of different plant species to glyphosate.This will be the subject of a future post.
  • I am assuming in my comments that you are carefully directing your spray and are attempting not to contaminate your plants. These paragraphs are about the degree of care that you need to take.
  • If you spray a large plant thoroughly with glyphosate it will usually be severely damage and in most cases you will kill it. Because of the translocated nature of glyphosate, if killing is your intention, it is not necessary to completely cover all of the leaves, but the better the cover, short of ‘run off’, the more successful you will be.
  • If a large plant only receives a little unintentional drift the proportion of leaves that receive herbicide is small. Such plants will often have sufficient resources to be  undamaged. The corollary of this is that small plants are at much greater risk.
This large eupatorium and helianthus will not be damaged at all by a tiny amount of drift
  • Although I have emphasised in previous posts that it is no use zapping a perennial weed as it emerges when most of the weed’s volume is the roots in the ground, do not be complacent  and assume the same is true of your emerging herbaceous plants. Although it will not kill them, many herbaceous perennials are extraordinarily sensitive as they emerge from the ground. Plants such as Aster novae-angliae, A.amellus, A.frikartii and phlox are liable to be checked and show unacceptable chlorosis.
I have been caught out and damaged  some of the shoots on this aster novae-angliae 
I am thoroughly ashamed of myself. The level of damage on this phlox is completely unacceptable. I must have been somewhere else when I sprayed! Perhaps it was windy?
Even after this desecration, from June the flowers were magnificent!
  • When young plants are making luxuriant active growth and their tissue is soft they are more likely to be damaged. For many delicate green plants including most vegetables, it is never safe to spray among them. I am particularly cautious when spraying my borders in spring and early summer. I am less careful in summer and by late autumn when plants start to become senescent I become positively complacent.
I am particularly careful when I spray at this time
  • Senescence is when plant tissues start to age and leaves turn yellow and brown. I find that damaging translocation of glyphosate to the roots is now minimal.This is at variance with some conventional opinion where ‘old generation’ hormone weedkillers were found to work best when applied to leaves of plants just before they senesced and started to translocate stored food reserves to the roots. Personally I have never found senescent plants to be particularly vulnerable to glyphosate. I find many herbaceous plants start to become more woody late in the season with fewer leaves at the base and I can less carefully spray. With plants such as my asters and phlox which are so sensitive in Spring, by late September provided I direct my nozzle downwards and close to the ground I find I can safely spray very near to a clump - even below the leaf canopy where I cannot see the weeds!

It is late September, my phlox has been flowering for all of three months. I am completely confident I can safely spray these grassy weeds if the spray head is low
I tend to direct my comments to herbaceous plants which are my passion. The same principles apply to shrubs which in general are more easily managed under a glyphosate regime. Because shrubs have a woody base and often a high canopy of leaves they are less likely to be damaged when you carefully spray. Spray that alights on mature bark of a woody plant is unlikely to be absorbed. This does not apply to green basal shoots on woody plants, nor to the green stems of roses. 
Some unexpected shrubs such as thuggish cultivated blackberries and hybrid berries or ornamental elderberry are surprisingly sensitive. I know to my cost!
When deciduous shrubs have dropped their leaves it is particularly safe to spray. 

Mingling. A few mixed messages
I am told the next horticultural fashion is ‘mingling’. You can see from the mingle-mangle below that much to Brenda’s scorn, I have a liking for mingled plants! Here in Bolton Percy cemetery you see a ground cover of Lamium galeobdolon, pulmonaria, ornamental strawberry and infiltrating wild garlic. For thirty years it was under very heavy shade as you can see  from the sculptured stumps of previously very large trees.

The patch can be used to illustrate a few points about using glyphosate.

A mingle mangle
1.If you start by killing all perennial weed the canopy of your plants does much of the the work of controlling weeds from seed.
2 It would be impossible to spray within clumps without causing damage, but the margins of clumps can be sprayed to keep weed free and to halt the spread of over-vigorous plants.
3. If I decide in a few years time that the garlic is becoming too much I can carefully spray the garlic when it is at its most luxuriant before it flowers (it would take all of two minutes). There might be some collateral damage but my vigorous ground cover will quickly grow back and nobody will notice.

The most significant timing of all  
I am afraid it is a council of perfection but where achievable it is really important. The best advice you can give anyone with a new garden or allotment is to get rid of the established perennial weed before they start.


  1. Roger, I have a few wee areas around my garden that I do use Glyphosate and find I use it exactly as you are describing.
    I've a persistent patch of brambles 'mingling' with willowherb- in an area that is difficult to access and find that by waiting until the stems are a good size and plenty of leaf - a good dose kills the willowherb but only knocks the brambles back. I do hope that I will eventually eradicate the brambles this way.

    1. Yes, Angie, brambles are more resistant to glyphosate but you will eventually get rid of them. Some gardeners use brushwood killer which is quicker. There are more dangers of short term residues in the soil damaging adjacent plants but coarse hedges and trees are unlikely to be harmed if you direct the spray just on the brambles!

  2. The advice about spraying where limnanthes have died back will be useful as we have a huge bed of them growing under roses on the plot which have lots of couch grass mingling in.

    1. Ideal, the couch will be at its most susceptible at that time
      I don't think those folk promoting the concept of mingling were referring to weeds, Sue and Angie, but the word certainly is most appropriate to weeds!

  3. Very useful blog, i use glyphosate in this way every year and get great results. I bought mine from and they gave me a really good deal. i look forward to reading you next blog.

  4. Milkweed is good for many things, including treating warts

  5. Any views help with lesser celandine at what point does spraying stop working before summer dormancy currently using glyphosate.

    1. Oh Matt, killing beautiful celandine!
      I jest, of course it can be a very difficult free seeding weed that survives with its tuberous root from year to year. I love it in my cemetery gardens but Brenda has banned it at home!
      Here in York it would be now at the beginning of April exactly the right time to spray with a strong mix of glyphosate - top end of the recommended range. The foliage is large and absorptive now. You will only really get one bite of the cherry as it will probably not make any regeneration before natural dormancy.
      Should your celandine be in grass or not growing amongst herbaceous plants you might still get some root absorption with lawn weedkiller (I use MCPA) with a later spray before it becomes completely dormant.

  6. Hello,
    I love your blog, it is really helpful to a weed control newbie. I have one major problem in my garden-bluebells of the Spanish variety I think. I have dug hundreds up but fear that many will return as it is often difficult to get all of the bulbs as they are so deep, or simply hidden. The RHS website says glyphosate is ineffective. I wondered if you know of there is any way to use it to eradicate them?
    Any guidance would be much appreciated.


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