Question: Why use turf on some sections and hydro-sprayed seed on others?
@Northern Light , please illuminate us!
Lets start by asking/answering Why Hydroseed?
Answer: Cost - its about 2/3 cheaper than sod, all other things being equal.
So why not hydroseed everything?
Answer: Hydroseeding will not give you an instant lawn, especially not in winter......it takes several weeks, in spring-like weather. Hydroseeding does carry some additional risks, as with any seeding program, it can be more subject to erosion risk on slopes (this is mitigated by putting mulch in the slurry that has adherent properties), and there is also a risk of geese eating your lawn before it even sprouts!
Also, hydroseeding is often not warrantied by the industry, where sod usually is covered.
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Summary:
Hydroseeding is cheaper, but slower, and slightly higher in risk.
Sod is instant lawn with better guarantees, but a lot more $$$
Looking at the pattern.......this is just my guess..........It looks like they went for sod on the busier roads/sides for instant gratification, and hydroseeded the balance.