Overseeding and transition. 1

A guide to overseeding and transition turf.

I guess the first question to answer is why overseed and transition turf? There are three reasons:

• To protect the underlying warm-season turf (couch/Kikuyu) which goes dormant and doesn’t grow in cooler weather;
• To give a good colour and
• To manage shaded turf. In many modern day stadiums shade causes the couch present to struggle. Overseeding with a cool season grass means the surface is playable when it is under the most use.

The key to successful overseeding and transition is timing. If it is done properly it protects the underlying turf and has a great colour over the winter months. Although a cheaper option, the use of turf pigments doesn’t protect the underlying dormant turf.

During the winter sports season, the lush green playing surfaces that you see on Australian TV are all overseeded with perennial ryegrass seed. So if you have warm-season turf then this article is directed at you if you want it to look good all year round.

The decision as to what variety to use and whether Continental or Mediterranean germplasm is one we will leave to you. However, this recent blog on NZ vs US ryegrass may help. 

Timing of overseeding and transition.

it’sThe timing of overseeding and transition of your turf is driven by soil temperatures and seed choice. Poor timing l the success of any work but if it’s coupled with poor seed selection, it can be a recipe for disaster. Also be aware that if you use pre emergent herbicides make sure these are not still active when you seed. 

For both overseeding in the autumn and transitioning in the spring correct timing is important to ensure both operations go smoothly. Growth Potential allows you to accurately determine when this should be carried out. It takes into account the temperature requirements of couch and ryegrass. Couch being a warm season grass it grows best in the summer months. In contrast ryegrass that grows better in the autumn/ winter months. The latter time is when couch can tend to take on a brown appearance as it goes dormant.

Using the Growth Potential for the Parramatta climate during overseeding and transition.

The image above shows growth potential for Parramatta in NSW. However, it can be applied anywhere climate data is available. In this particular situation overseeding after mid March is when the ryegrass will be growing best. For transitioning the ryegrass out this should occur after October for exactly the opposite reasons. Unfortunately due to use constraints you may well not be able to carry out this work at the ideal time.

Overseeding and transition – Seed Selection.

Choosing an appropriate turf seed is important for any success of overseeding and subsequent transition work. Generally either annual or perennial ryegrass is used. For the home gardener I’d always buy wholesale as then you know what you are getting. The key to seed selection is reading the label but there are a few tips you can use to make sure you are getting what you are paying for. All commercial seed should come with a seed certificate showing its purity, how old it is, germination percentage, and weed seed content.

Bare in mind commercial pricing of annual ryegrass is around $4.5/kg, perennial ryegrass is $6.50/kg and uncoated Kikuyu seed is around $60/kg.

The following applies to all seeds but we are using Kikuyu seed in this example. Two things to immediately look out for are the physical contents and if it is coated. Terms such as “contains nursery cover for faster establishment” should also set alarm bells ringing. In the “home garden world” a box of Kikuyu seeds, doesn’t just contain Kikuyu seeds.

Kikuyu seed.

Hortico Kikuyu blend (500g is approximately $25/kg) contains Kikuyu and annual ryegrass with the latter incorporated to ensure faster establishment (and keep the cost down!). If you look closely it only contains only 7% Kikuyu seed!

Munns Professional 2.5kg Kikuyu Lawn Seed Blend is around $65 and contains Kikuyu plus perennial ryegrass and some other bits and pieces (Fertilizer and wetting agent). In reality, it contains 90% perennial ryegrass and only 10% Kikuyu. The marketing as “germination in 5- 7 days” is purely to do with the ryegrass component and nothing to do with the Kikuyu component.

Landscape Range Turbocote Kikuyu Lawn Seed is $81/kg and contains 100% coated kikuyu. Bearing in mind that coating can be up to 50% by weight in reality this means that it could equate to $81 for 500g or $162/kg…

Quality of seed is important for overseeding and transition.

Commercial seed all comes with a seed certificate showing germination percentage, weed seed content, etc.

Purity:

This is the percent by weight of each seed component in a mixture as expressed on the seed analysis tag. Purity is not necessarily an indication of “seed quality” but rather is a measure of “seed quantity.” It is important to recognize that not all pure seed is capable of growth and surviving to maturity.

Germination Percentage:

Germination is expressed on the seed label as the percentage of pure seed that is capable of growth. This value is determined in near “ideal conditions” in terms of temperature and moisture for germination compared to typical field conditions. Germination declines with the age of the seed and therefore seed older than 9 months to 1 year may be less viable than fresh seed. Only the freshest seed of the highest purity and germination should be purchased.

Weed seed:

The percent by weight of all seeds in the container which have not been identified as either pure seed or crop seed. This value should ideally should be 0.5% or less.

Inert matter:

The percent by weight of all material in the seed container that will not grow. This value should be as low as possible.

Example of seed certificate

Example of labelling

But we digress! What about the selection of overseeding grasses such as ryegrass? To start with avoid the flash brochures. Your paying for those either in the price you pay or the quality of the seed that you purchase. Instead, look for independent data and two good sources of this are NTEP and ANTEP. This is the key reason that if you buy commercial seed you can see what varieties you are buying. It’s like buying a car. Not all cars are the same. Colour, engine size, fuel economy, etc can vary depending on the manufacturer. With grass seed, colour, establishment rate, disease tolerance, and quality of cut amongst others can all vary. These independent trials rank how varieties perform so you can make an informed decision.

So you have trials looking at specific characteristics. Many of these may or may not be relevant to a lawn owner or turf manager. However, that isn’t the entire story. In the case of ryegrass (the most commonly used turf seed variety for overseeding), you have a further complication of whether it is an annual or perennial.

oversowing warm season turf research plots.

Overseeding rate

This is also important. We have often seen rates of up to 5kg/100m2 recommended which is way too high. In reality for the home garden you are overseeding generally for aesthetics and so rates of 2-3.5kg/100m2 are fine. For overseeding sports grounds, 3.5Kg/Ha is fine. Realise that at some stage you are intending to remove this and so overseeding at 5kg/100m2 will only choke out the underlying turf and make it more difficult to remove when you decide to do so.

The image below shows a key difference between annual and perennial ryegrass.

Colour of overseeding grass.

Perennial ryegrass tends to have a much darker colouration coupled with increased wear tolerance, a finer leaf, increased disease tolerance, and an ability to withstand heat better. When you watch winter sports on TV and it’s a dark colour it’s highly likely to be perennial ryegrass.

Ryegrass for overseeding.

There are now a new generation of perennial ryegrass cultivars on the market. These can regenerate after use. The images above and below show “Torsion LS” after divotting and the recovery afterwards. Obviously, if it is subjected to high wear every day it isn’t going to get time to establish or recover.

If you want a successful result from your overseeding and transition don’t do this!

The following practices are NOT a good idea if you are seeding in the autumn.

• Aggressive scarification. This removes stolons which act as ‘’growing points’ when you transition the ryegrass out in the spring. 80-85% of these stolon buds are removed by aggressive scarification. Consequently the only place the couch can grow back is from rhizome growing points. As the couch has more stolon growing points than rhizomes per square metre it limits spring recovery of the couch.
• Overseeding too early. This allows the couch to grow back and uses limited food reserves in rhizomes, and crowns. The result is in even less is available for spring transition back to couch. The couch will continue to compete with the overseeded ryegrass giving a poor ryegrass establishment and poor spring transition. As a rough guide overseed when the night time temperature is around 12°C.

But you can consider this in your overseeding and transition programme:

Use a PGR (Growth regulator). If the grass to be overseeded (couch/Kikuyu) is still actively growing and you want to overseed, slowing down the couch/Kikuyu can be very beneficial. Trinexapac ethyl sold under a variety of brand names such as Amigo 120 works well for achieving this but you MUST raise the mowing height before application. A high percentage of this is taken up within an hour and it takes around three days to fully kick in. Do NOT scalp the surface before applying trinexapac as this needs to be taken up by the plant. Also don’t leave any clippings on the surface before application because it can’t reach the plant then.

A suggested overseeding and transition programme would be:

Mid-summer.

If thatch is 50mm or more vertically mow to remove, aiming to carry this out no closer than 6 weeks before seeding to allow recovery.

35 days before seeding.

Cease all N applications and increase K

12 days before seeding.

Raise the mowing height by 35-50%. Decrease irrigation by 25% to force the couch/Kikuyu to store carbohydrates and also slow up growth. If you are going to apply a PGR carry on irrigating.

5 days before seeding.

Now is the time to apply your PGR.

1 Day before seeding.

Mow the turf to its original height (same height as three weeks previously). This scalps the grass removing the upper leaves. Drop the height another 30-50%. This will in reality be the only decrease in height although you will have ‘’double scalped’’ the turf. The result is a semi-upright stolon with one leaf on it. This makes it easier for the seed to get down to the soil and the stolons will be in better condition for the spring. Using some of the smaller seed variety ryegrasses will also help with the success of seeding.

Day of Seeding.

Overseed in two directions. Drag a matt over the surface and roll if possible. Ensure good soil seed contact.

Next Day.

The aim is to irrigate on a light and frequent basis. Don’t apply more then 6mm per day (depending on date seeding actually takes place), with say 3-5 irrigation events over the day. Don’t irrigate past 4pm as late irrigation will tend to encourage disease.

First Mow.

Mow the ryegrass when its 12-17mm in height with a cylinder mower

First Fertilization.

Apply a starter after the first mow. This reduces the chances of any granular fertiliser being removed.

Make sure you get even coverage when seeding!

Transitioning out.

There are two ways of doing this. The first is too simply to reduce the mowing height and cut off the water. Then let the heat and nature take its course. The cool-season grass becomes stressed in the heat and dies off. This is the principle behind using annual ryegrasses to overseed.

The issue here is that if the temperature doesn’t get hot enough, the grass that you use to overseed will not stress out and die off. Instead it will continue to out-compete the warm-season turf. The result is the cool season grass persists at the expense of the warm-season turf.

Chemical transition

The second option is to chemically transition out the ryegrass. There are several options for this such as the sulfonylureas. Ideally you want to achieve an accelerated transition with no green-brown-green.

Instead you want green-green.

Tips for accelerating spring transition of ryegrass.

• When do you accelerate transition?
  • Minimal root growth < 10°C. soil temp.
  • Optimum root growth 24-29°C. soil temp.
  • Shoot growth ceases < 13°C. air temp.
  • Optimum shoot growth 27-35°C.

Need night air temps. of around 16°C.

Sulfonylureas

To get the best results from these make sure the soil temperature is above 21°C. Ideally greater than 26ºC is even better (Hutto et al 2004). You must realise that you are trying to achieve two things. Firstly to remove the overlying cool-season grass and secondly to ensure that the underlying warm-season turf (e.g couch) is actively growing to fill in any thin or bare areas. This is why in the US these are referred to as late transition aids.

If the temperature is not warm enough then control will be very variable. You are likely to end up with areas showing clumpy ryegrass and bare areas where the underlying warm-season turf has not filled in. Over the 2021 season, it was chemical control that lead to “hit and miss” results and a waste of time and money.

All of the chemicals below are in the sulfonylurea chemical group These all behave in a similar fashion so the principles are the same them when you use any of these.

Clumpy Ryegrass

Clumpy ryegrass is more difficult to control than overseeded perennial ryegrass.  A well-timed herbicide application in early spring should provide nearly 100% control of perennial ryegrass within 21 days of treatment. The same herbicides only provide 50-60% control of clumpy ryegrass over the same time period.

Speed of Transition

The speed of transition also plays a major role in deciding when to apply these. Aim to apply a faster acting chemical as late as possible into the spring unless dormant looking couch is acceptable. You should apply slower acting options such as Kerb, which can take up 8 weeks to work much earlier in the season.

Herbicides from slowest to fastest removal of perennial ryegrass: Kerb®, Tribute®, Coliseum®, and the Monument herbicide alternative, Recondo herbicide®.

In general, warmer temperatures usually increase the transition speed, whilst you should avoid applications at soil temperatures lower than 10°C. Research has shown that if soil temperatures are below 18°C there is a reduction in control with Tribute® being more sensitive than for example Monument® liquid herbicide.

Trial work on sulfonylurea use for ryegrass transition.

Work by Envu shows some results when you use different SU herbicides for turf type ryegrass transition out of couch grass.

Non ionic surfactant.

The addition of a non ionic surfactant is essential to ensure that the SU’s work properly. This ensures excellent contact between the leaf and chemical. Rate is 0.25% by volume.

Control of both clumpy ryegrass and tillering crowsfoot is often improved by adding methylated seed oil (MSO) or urea  ammonium nitrate (UAN) to sulfonylurea applications. These improve penetration and uptake, which is important on older established plants.

However, studies show that by adding MSO + UAN to Tribute®, control of clumpy ryegrass can be achieved in cooler conditions, as long as daytime highs are 20°C plus.

Water pH

Usually, a high spray water pH is bad. However, herbicides in a class known as sulfonylureas generally work better in alkaline (high pH) water. For example, registered sulfonylurea herbicides state that if water pH is 5.5 or below use a buffer to raise the pH to near 7.0. Work out of Spain indicates that the addition of Delfan Plus can actually improve sulfonylurea efficacy as it is increases the tank pH and also counters any non target pyhtotoxicty.