The Robinson Garden at Earlscliffe, Baily, Co. Dublin, Ireland

Herbaceaous Plants and Simazine
Plants and gardens of the Caribbean
Article on Wollemi Pine

Some herbaceous plants can stand simazine

David Robinson has useful information for gardeners wishing to cut down on hand weeding © Muriel Robinson 2008

Gardeners who use simazine to control weeds know that it can be used safely on most woody garden plants but that there is a risk of damage to a large number of herbaceous perennials. Some popular kinds, such as Phlox and Verbascum are so susceptible that the normal recommendation is to keep simazine away altogether from the herbaceous border and from mixed plantings of shrubs and herbaceous species.

This is unfortunate because a garden composed only of trees and shrubs lacks the exuberance and lushness of herbaceous perennials during the growing season.

Since 1958, I have relied heavily on simazine to keep my collection of trees and shrubs in a relatively weed free condition in gardens at Loughgall, Malahide and now on the Howth peninsula. Over the last 40 years I have noticed that some herbaceous plants, succulents and species with bulbous or fleshy rootstocks show a good degree of resistance to simazine. This means that they can be used with shrubs to help soften their sometimes rigid outline. Some plants that can be used for this purpose are listed in the table.  

Some herbaceous, bulbous, rhizomatous and succulent plants that tolerate simazine 

This list is far from complete

When simazine is applied to soil it is adsorbed by clay and organic matter and is released slowly, killing germinating seedling weeds over a period of many months. The normal rate on a medium loam soil is 1 ounce of the 50% powder to 100 square yards (34 g/100 square metres) or 1 fluid ounce of 50% liquid simazine to 100 square yards (34 millilitres/100 square metres).  The simazine needs to be well mixed with water and applied uniformly.  The amount of water used is not important but will usually range from about 0.5 gallon to 2 gallons/ 100 square yards (5 – 22 litres/100 square metres).

Because of differences in clay and organic matter content, the amount of simazine used should be reduced slightly on light loam soil and should be increased on heavy loam. Simazine should not be used on very sandy soil low in clay and organic matter. On this type of soil, there is a great risk that the herbicide will move rapidly downwards, come in contact with a large part of the root system and cause damage.

Simazine is almost entirely taken up by plants through the roots and negligible amounts only enter through the leaves. This means that wetting the leaves of garden plants during spraying will not cause any damage, provided the correct amount is used. I find that the most efficient way to use simazine in mixed plantings is to use a flood jet nozzle which enables the spray to be applied in swathes 2 metres wide. Holding the nozzle steady while walking at a slower than normal walking pace, the herbicide is then sprayed evenly over soil, shrubs and herbaceous plants. Although some of the herbicide will be retained on plant leaves, rain will eventually wash it off unto the soil. Paradoxically, gardeners who apply simazine in rings around the base of garden plants, carefully avoiding wetting the foliage, run greater risk of damaging plants. Herbicides used in this way cannot be applied uniformly to the soil surface. 

When simazine is used at low rates for the selective control of germinating weeds in gardens, practically all the chemical is retained in the top 5 cm layer of soil. This band is the main germinating zone for annual weeds. Large seeds, germinating from lower depths and perennial weeds with fleshy rootstocks and underground food reserves, are not affected by normal rates of simazine. For this reason the herbicide should only be applied to soil surface that is clean and substantially free from perennial weeds. If the soil is very dry, the herbicide will be retained at the soil surface and will not penetrate the soil to reach the main germinating zone of weed seeds.

Why do some plants stand up to simazine while others are susceptible? A number of factors are involved. Some plants, such as Libertia and Cyclamen hederifolium, appear to be able to break down and detoxify simazine as they absorb it. Others, including many bulbs, do not absorb simazine from the soil because, planted in the normal way, their main root system is below the soil layer reached by the herbicide.

Even when the correct rate is applied, simazine may cause slight yellowing of the foliage of some plants. This effect is usually temporary and, in any case, is likely to be less detrimental to plants than the unseen damage caused to plant roots by hoeing and hand weeding.

Because simazine controls such a wide range of weeds, it was often used in the past at 20 times the normal dose to give total control of all weeds on hard surfaces. On roads and footpaths there is no clay or organic matter to adsorb the chemical and so it was washed rapidly into drains and rivers and contaminated ground water. Simazine should only be used at low rates on soil, not concrete or stone aggregate, to avoid pollution. 

This article has been written to give general guidance only. Soil conditions and seasons vary widely, so that gardeners intending to use simazine should test the herbicide n a small area initially to make sure that it will give satisfactory results under their local conditions.

This page was last updated on 24-Feb-2008 .