Scarifying and Dethatching

What is Thatch?

In New Zealand, we have a number of different lawn species and each has benefits and drawbacks. If you have anything other than a pure Rye grass lawn it will need dethatching or scarifying at some point. Generally the finer the lawn the more care it will require. These fine lawns can look amazing with the right care but do build up layers of thatch quite quickly.

Grasses propagate by seed with some also producing side shoots or runners. These side shoots help the grass spread and are just above the soil surface (stolons) or just below it (rhizomes). The finer lawns have an abundance of stolons and rhizomes which gives them the thick, lush appearance.

At some point, the parent grass plant and some stolons and rhizomes die to be replaced by younger stronger plants. The problem is that the growing part of the grass plant (crown) together with the stolons and rhizomes are slow to rot or decay. And because they don’t decay and compost into the soil like normal plants would, they start to build up, layer upon layer forming what is commonly called thatch.

The trade-off for having a beautiful lush lawn is the amount of thatch removal required. The problem will not go away and the longer it is left the bigger the job becomes.

In extreme cases, where layers of thatch are left to build up over the years, the lawn will eventually stagnate and start to die back. So it’s best to deal with it early!

Dethatching vs Scarifying

There are a number of different machines on the market that are capable of removing the thatch or you could tackle it by hand, depending on the size of the lawn and the extent of the issue. However, be warned that attempting to remove thatch with a rake or a rolling lawn scarifier is a tedious and backbreaking task and not recommended.

Power rakes or lawn scarifiers are machine powered and use steel blades to cut out thatch. When this process is performed on the surface of the lawn it is referred to as dethatching. This is a lighter, less invasive option than scarifying. Once completed the result is a firmer lawn which allows a free passage for air, water and nutrients to the roots. 

In severe cases, a more intensive course of action is required. By lowering the blades on the scarifier and letting the machine blades cut into the soil, you will open the soil surface and make an ideal seedbed for introducing new grass seed to your lawn. This process is commonly referred to as scarifying in New Zealand. 

Lawns are living organisms and will by nature always have thatch, it is the control of the thatch that is important.

The Right Amount of Thatch

Too much thatch is considered to be in excess of 15mm. When this happens the lawn starts to feel spongy underfoot. As well as feeling like a sponge it can also act like a sponge withholding valuable water, air and nutrients needed by the roots of your lawn.

Conversely, a little thatch, somewhere in the region of 5mm is beneficial. It is not sufficient to hold valuable nutrients away from the soil and grassroots, but is enough to cushion and protect the crowns of the grass plant and insulate the soil from the drying effects of wind and sun.

If the thatch is left unattended it will build to the point where the grass starts rooting into it in search of the water and nutrients trapped in the spongey layers. At this point, it becomes critical to dethatch the lawn or run the risk of losing it altogether.

Dethatching or scarifying a lawn is very beneficial for the lawn but needs to be done correctly. Incorrect dethatching can leave a lawn looking terrible in the short term. Though it will recover and look better than ever, achieving this in the shortest possible time is the ultimate objective.  Incorrect timing of the procedure or misreading the conditions can leave the lawn looking worse for months afterwards and may cause damage to the healthy new shoots.

Time the programme according to the ability of the lawn to recover. This means there must be plenty of warmth, sun and rain and minimal cold, heat or drought. If the lawn isn’t growing well before scarifying then it won’t grow well afterwards leaving the lawn looking terrible for a few seasons before it recovers.

So when is the Best Time to Dethatch?

Light de-thatching or scarifying is best to be done in spring and or autumn. Spring generally means sometime in September when the temperatures start to warm up and the growth and recovery rate increases. Autumn would generally be late March or April as the rains start but before the frost sets in.

It is best to leave the heavy scarifying to autumn. The intense cultivation will leave the lawn thin and exposed in many places and the heat of summer will damage the lawn. The cultivated soil is not only a great seedbed for new lawn seed but also weed seeds that may be floating around. By holding out on this intensive treatment until early autumn you will radically reduce weed infestation. Having said that, there are a few exceptions when spring is the best time:

·        It is better to rake or scarify in spring if you missed the autumn window due to bad weather and rather than putting it off from autumn to autumn just deal with a few weeds.

·        If the area to be scarified is shady. Shady areas will thin over winter and thicken up from spring onwards. Scarifying in autumn will just make things worse.

·        If the area is under trees. The shade from trees plus the autumn leaf drop will reduce grass health and vigour but in spring, (late August through September) the trees should be bare enough to allow light onto the lawn enabling good growth and maximum opportunity for new grass seed to germinate.

Lawn preparation for Scarifying or Dethatching

Planning ahead is important. The grass needs to be short and dry for raking or scarifying so bring the grass height down gradually over a week or two beforehand. This doesn’t shock the grass whilst also allowing air deeper into the turf helping dry the grass.

The final bit of preparation is to have good soil moisture so that the lawn recovers as quickly as possible afterwards. If there is too much (soft ground) or too little (dry hard soil) you need to either hold off until the soil conditions improve naturally or help them improve.

What to do after Scarifying

After any intensive treatment such as scarifying, it is important to assist lawn recovery. This is easier when the timing is done correctly and nature is helping by providing some warmth, sun and rain to help the process along.

At this point, it is recommended to over sow the lawn with a selected grass[I1]  seed. If the damage is patchy then a light oversow into those areas may be all that is needed. However, if a thorough scarification or de-thatching has been carried out it’s risky to expect a full and uniform recovery so get some grass seed into the whole lawn.

 Start lawn recovery

Start by fertilising the lawn, a fertiliser with a high potassium content is recommended. This helps the grass recover from the stress of the scarifying and will help protect against disease. LawnFix  carries a wide range of fertilisers that have the perfect balance of nutrients to get lawns back on track after scarifying.

Finally, the lawn needs a good amount of water to speed the recovery along. Rain is always best but can be very erratic in New Zealand, that’s why a well-designed and professionally installed irrigation system is helpful. Consistent, even watering of the lawn will bring it back to prime condition in the shortest time possible.

For some more personal advice, contact LawnFix on 0508 LAWNFIX (529 6349) or visit Our Services page for more information on what we can offer you.