Yellow grass is a problem that can strike all year round, but particularly in summer, due to a combination of factors. Grass often pays the price for poor maintenance (irrigation, mowing, fertilisation etc.).
What is the best treatment for a yellowing lawn? Regardless of the source of the damage, typically the best solution to regenerate a lawn is reseed the damaged areas in autumn or spring. This is why it is important to act early: today we look at the main causes of a dry lawn, before giving you recommendations for solving and preventing the problem.
Dry lawns and irrigation
The exceptionally high temperatures and dry weather during the recent summer clearly demonstrated how lawns turn yellow in the absence of water due to lack of rain or irrigation. The water requirement of every lawn varies depending on the type of grass, the area where you live, the season, the climate and the soil characteristics. Here in Italy, daily rainfall fluctuates between 5 and 7 l/m2 from north to south.
Only water it every 2–5 days, but do so thoroughly to make up for the days of drought. Ideally the grass shouldn’t get wet too often. This is because a moist environment is the ideal habitat for fungal diseases, which are another typical cause of yellow patches on the lawn. Among other things, this cyclic irrigation system stimulates the grass to develop deep roots and therefore become more resistant to heat.
For the same reason (avoiding constant moisture) water the lawn early in the morning, so that it has time to properly dry out.
Yellow lawns and fertilisation
Lawns need nutrients, just like the plants in your vegetable patch and garden. If fertilisation is insufficient or even excessive, the result can be a yellow lawn (a classic symptom of nitrogen deficiency).
In principle it is advisable to fertilise the lawn 4-5 times a year: when vegetative activity restarts in late winter (March), late spring (May), between late spring and early summer (mid-June/July), and in early and late autumn (September and December).
Each fertilisation must be done with the right type of fertiliser and dosage for the time of year and intended purpose. Generally speaking you should use a slow-release NPK fertiliser — consisting of nitrogen (N), phosphorus (P) and potassium (K), although in summer it is best to avoid fertilisers containing phosphorus. Instead, you can use a fast-acting fertiliser to revive a lawn weakened by nutritional deficiencies.
Applying excessive amounts of fertiliser can burn the grass or turn it yellow: to remedy this, collect up as many of the granules as possible from the ground, then water the grass copiously to dilute the concentration. This solution also applies if you inadvertently administer the wrong product. In addition to sticking to the recommended dosage, distribute the fertiliser evenly: a fertiliser spreader cart can be useful for this.
Dry spots and fungal diseases
Various diseases are caused by fungi and can attack lawns almost all year round, especially between May and September. Depending on the parasite and its lifecycle stage, the dry patches will be large, more or less rounded and either whitish grey, yellow or brown.
Fungal disease should be tackled by applying a broad-spectrum fungicide or, better still, a specific or “narrow-spectrum” fungicide (if you can identify the fungus responsible, such as Pythium or Rhizoctonia solani, to name two of the most common). Strictly adhere to the recommended dosage: fungicides can be toxic to people, pets, soil and the lawn itself.
Certain conditions promote the spread of pathogenic fungi, overwatering being the prime culprit. Other major causes of fungal disease include nutritional imbalance, improper mowing, thatch and extreme climatic conditions (humidity, excessive heat, frost). When faced with a sick lawn, it is advisable to work out which factor or factors made it so vulnerable, in order to avoid any recurrence of the attack.
How to prevent a lawn from yellowing
Obviously it is better to prevent grass from yellowing than to treat it afterwards and then reseed to revive the damaged patches.
So to keep your lawn healthy and prevent it from yellowing:
Apply moderate watering, without leaving the grass moist for excessive periods, as explained above.
Fertilise using the methods and timing recommended above: by the way, potassium helps plants to combat diseases, drought and cold but, as always, don’t go overboard with it.
Mow the grass to the right height.
Grass that is cut too short is easily stressed out. Each season has its own ideal height and the hottest and driest months are the most stressful (here you can find tips for mowing the lawn in summer).
Has your lawn turned yellow after being mowed? Make sure that the blades of the lawnmower or garden tractor are well sharpened so that they cut cleanly; otherwise they will tear the grass, causing the tips to yellow. The same is true when using a wheeled brushcutter. On the other hand, if you use a brushcutter you should install the appropriate type of line: if it is too thick it will shred the grass, rather than cutting it cleanly.
Thatch acts as a barrier preventing light, water and oxygen from reaching the soil; it increases water stagnation and the risk of fungal diseases; and it reduces the effectiveness of fertilisation and reseeding. So, keep your lawn clean: limit mulching to periods when the grass decomposes more easily (in the hottest months) and make use of a blower to rid your lawn not only of leaves (one of the gardening jobs that need doing in the autumn), but also pruning debris, sawdust and so on.
Periodically aerate the lawn (hit the link to find all the steps you need to follow). This is often an overlooked task that should be done with a scarifier to specifically target thatch (as well as moss, which similarly promotes moisture build-up).
Another possible cause of yellow grass is failure of recently laid turf to take root (this may happen with very small clods, or along the edges). Here you can check all the steps for laying rolled turf the right way, or check out this step-by-step guide to lawn seeding.