When to Plant Grass Seed in the GTA (and When It's a Waste of Money)

The window that works in the GTA is mid-August to mid-September. That is the Guelph Turfgrass Institute's answer for Ontario: "Generally, the best time to establish a lawn from seed in Ontario is mid-august to mid-September". Environment and Climate Change Canada's climate normals for Toronto Pearson, the long-run averages for that station, show why. Mid- to late spring is a real second choice. June, July and early August are where the money goes. Late-fall dormant seeding works, but it often fails.

Germination runs on the seedbed, not the calendar

Purdue's turfgrass program publishes germination figures, the temperatures at which a seed sprouts. Two conditions come with them. Almost everyone drops both. First, they are air temperatures, "which would be almost identical to that … on the surface of the soil barring radiation effects". So you measure the top centimetre the seed sits in. Second, the pairs are not a range. Purdue is plain: "Temperatures separated by a dash indicate an alternation of temperatures. The first number is for approximately 16 hr and the second for approximately 8 hr". So: cooler at night, warmer by day.

The °C conversions are ours. Purdue lists no emergence times, so those come from Penn State: Kentucky bluegrass is "slower than for most other turfgrasses", perennial ryegrass "germinates rapidly". Bluegrass starts moving in cooler soil than ryegrass. It then takes twice as long to show.

Growth needs a third set of numbers, lower again. UMass Extension puts the ideal air range for cool-season shoot growth at "approximately 65 to 75 degrees Fahrenheit" (18.3–23.9 °C). The ideal soil range for root growth is lower still, "approximately 55 to 65 degrees Fahrenheit" (12.8–18.3 °C). For a working floor, the University of Saskatchewan is clearest: "Over-seeding is best done when soil temperature is at least 10°C for germination. Since soil is cooler than air temperature, wait for daytime temperatures of 15°C for best results".

So buy a soil thermometer and push it in to seed depth. The 1991–2020 normals page for Toronto Pearson has no soil reading at all: search it for "soil" and you get nothing. The air numbers below stand in.

What the GTA year actually looks like

Environment and Climate Change Canada's 1991–2020 normals for Toronto Pearson (climate ID 6158733): air temperatures read at screen height at the airport in Mississauga.

August's 15.8 °C overnight and 26.3 °C by day comes closest to the 15/30 pattern Kentucky bluegrass wants. September is cooler than ideal but still works. April's 1.9/12.0 sits below every figure in the table, for every species.

Frost, same station. The average last spring frost is April 27. The average first fall frost is October 20. That leaves an average frost-free period of 174 days. Those averages use 23 of the 30 years. The odds rows use the longer 1941–2020 record. A 50 % chance the last spring reading at or below 0 °C lands on or after May 4. A 50 % chance the first fall one lands on or before October 9.

Why mid-August to mid-September wins

Three things line up at once, once a year. First, the soil still holds August heat while the air cools. Penn State's version: "soils tend to be dryer and warmer during late August and early September than in spring". Second, weed and disease pressure has dropped. Purdue prefers "seeding from Aug 15 through Sep 15 because weed and disease pressure are minimized". Third, the young grass gets two cool seasons instead of one. Penn State again: "when planted in late summer, seedlings will have two cool growing seasons (fall and spring) to become established, whereas seedlings developing from spring establishments will be subjected to the heat and drought of summer, making survival more difficult".

UMass puts the window for most of Massachusetts at "approximately the third week of August until about the third week of September". Delay it, and "less cover and density going into winter can be typically be expected with each passing week that seeding is delayed". Delay further and "after the autumnal equinox, establishment projects become less of a sure thing and more of a gamble with each passing day".

Spring seeding: the honest version

Spring is fine for "bare patches or winter damage". The Guelph Turfgrass Institute adds one condition: "The soil should be thawed and not saturated with water because germination may not occur if the soil is too cold and damp". Penn State says "In Pennsylvania, optimum times for turf seeding are mid/late spring and late summer/early fall". Note mid/late. April's daily maximum is only 12.0 °C. And with an average last frost on April 27, an early-April seeding lands in a bed below USask's 10 °C floor.

The second problem is chemical. Pre-emergent herbicides kill weed seeds before they sprout. They stop grass seed too. Nebraska Extension says only one of them, siduron ("commonly sold as Tupersan"), will "still allow the grass seed to germinate". Crabgrass and foxtail need "several days of soil temperatures 55 degrees F or above" (12.8 °C) before they start. In spring you get weed control or new grass, not both. In September you get both.

Four ways a seeding wastes your money

1. Heat. Penn State's summary: "summer seedings are subject to heat, drought, and disease problems". Purdue kills the "it never came up" theory: "Most of our cool season grasses can germinate almost throughout the summer … Poor irrigation, diseases like damping off, and weed pressure are the most common causes of seeding failures". The seed usually does sprout in July. The seedling then dies.

2. No water, and in parts of the GTA no lawful way to give it. New seed is a daily job. USask says to keep it moist until the seeds sprout, "which can take 7 - 10 days depending on temperature, so daily watering is critical … You may have to water more than once a day if the weather is warm or windy". Then check what your city allows in July. York Region gathers its local by-laws in one place. Unless a municipality says otherwise, they run May 15 to September 30, on odd/even days matching your house number.

King is the only entry with no grace period at all. York Region's own caveat covers the rest: "Sod and seed watering details vary between municipalities".

The rest of the GTA is looser. Peel Region supplies Mississauga, Brampton and Caledon. Its outdoor water page sets no odd/even schedule, and the advice there ("water your lawn and garden before 10 a.m. or after 7 p.m.") is voluntary. Halton Region runs a level system instead. At the level posted when we checked (Level 1, "Careful use"), it only "asks residents and businesses" to follow odd/even days. Higher levels are backed by Halton Waterworks By-Law 79-19, which allows set fines and summonses. On new seed, Halton's answer is "Yes, until the sod is established".

Toronto has no standing schedule. Municipal Code Chapter 851 (consolidated to April 1, 2019) lets the General Manager of Toronto Water issue a water use restriction order. That order can carry "Prohibitions or limitations on watering of lawns, sports fields, grassy areas and golf courses" (§ 851-14E(1)). New seed is not automatically exempt. Under § 851-14F the General Manager may exempt "newly sodded or seeded lawns", or set lighter limits. That is the city's choice, not your right.

3. The wrong species for the date. UMass rates Kentucky bluegrass "among the slowest to establish relative to comparable cool season grasses". So a bluegrass mix belongs early in the window. Perennial ryegrass "offer[s] more flexibility in terms of later planting". But it "is among the least low temperature tolerant among available cool season grasses". A late patch of pure ryegrass can still die over winter. Swapping species to rescue bad timing is no fix: "adjustments in species selection in an attempt to make up for less than ideal timing are not recommended". The Guelph Turfgrass Institute's default lawn splits the difference: "a mixture of Kentucky bluegrass and Perennial ryegrass".

4. Seed onto unprepared soil. Thatch is the mat of dead stems under the green grass. Scattering seed onto it wastes the bag. USask's order of work: rake hard, de-thatch if the thatch is thicker than 2 cm, spread 1 to 2 cm of compost or good topsoil, then "Lightly tamp the area to ensure the seeds have good contact with the growing medium". Penn State adds straw on bare soil at "80 to 100 lb of straw/1000 sq ft … on level or gently sloping areas". Once the grass is 7 to 10 cm tall, USask drops watering to "once every 7 - 10 days at a rate of 2.5 cm of water".

Dormant seeding, honestly

UMass defines dormant seeding as "the planting of seed into a prepared seedbed after conditions conducive to germination have ceased for the season". The hope: the seed sits still all winter, then sprouts the moment spring turns mild. UMass is careful: "available research suggests dormant seeding may or may not be as effective as waiting for early spring seeding, depending on a host of factors". The main risk is a January thaw, "a warm spell during the winter or early spring that initiates germination of seedlings which are subsequently killed if bitter cold returns". That risk flips the usual species advice. Dormant seeding "often actually works better for slower-establishing species such as Kentucky bluegrass, and less well for faster species like perennial ryegrass". A winter with little snow cover "can also cause seeds to dry out, greatly reducing germination percentage".

The target is a temperature, not a date. UMass says to seed "when it is too cold for seed to germinate, but the ground is not yet frozen. Shoot for air temps consistently below 50 and soil temps in the low 40s or below". That means under 10 °C in the air and roughly 4 to 6 °C in the soil. Nebraska Extension sets the same bar: "Soil temperatures must be 40 degrees F or below to ensure seeds will not germinate" (4.4 °C). It caps the window too: "dormant seeding should be done no later than March 15th". In the GTA the window opens after the average first frost on October 20, once Pearson's normal daily average has fallen through November's 4.1 °C. UMass advises dormant seeding "only with the goal of encouraging greater density or filling in bare areas, as opposed to establishment of new areas of turf".

What the seed actually costs

Work it out for 1,000 sq ft. Islington Nurseries Stone Yard states 440 sq ft per kilogram, so 1,000 ÷ 440 = 2.3 kg. At 2.3 × $14.99 that is about $34 of consumer-grade seed. Striped Seed's 5 kg elite bag works out at $197.00 ÷ 5 = $39.40 per kilogram. Its own new-lawn rate is 1 to 2 kg per 1,000 sq ft. So 1 × $39.40 = $39 at the low end, and 2 × $39.40 = $79 at the high end. Sod for the same area is 1,000 ÷ 9 = 111 rolls, and 111 × $4.50 = about $500 before delivery. Queensville delivers only on orders over 40 rolls; below that you collect them yourself.

The seed is never the expensive part. The second attempt is: the topsoil, the rental, the weekend, and a summer spent watering a lawn that never rooted.