The borough is already a finalist for Britain in Bloom and volunteers are hoping it will be named one of the cleanest, greenest and most beautiful places in the UK.
Parts of Uxbridge, Ruislip and Eastcote have been assessed.
Hillingdon won a Britain in Bloom gold in 2015 and will discover before November whether it has got the top award again.
Judges braved the torrential rain to take a whistle-stop tour of the borough in recent weeks.
Accompanied by members of the Hillingdon Council's Green Spaces team, the judges visited the Rural Activities Garden Centre, Stockley Park, Uxbridge town centre, Eastcote House Gardens, Eastcote Station and Manor Farm along the way.
Judges Darren Share and Geraldine King from the Royal Horticultural Society said how impressed they were with the hundreds of volunteer gardeners who contribute so much, commenting that even the considerably wet weather couldn't dampen spirits.
Council officers said they were delighted to see the sunflowers shining brightly, even if the sun wasn't.
Entries will be marked against three key criteria:
Each finalist will be awarded a Gold, Silver Gilt, Silver or Bronze medal, depending on the standard reached. There is also an award for being the best in each category, for example, the village, town or city category.
Roger Burnett, Chair of the RHS Britain in Bloom judging panel, said: "Britain in Bloom really brings out the best in people and is a force for good across the UK. What started out as a civic pride campaign, has evolved into something that helps tackle some of our most pressing issues.
“Taking part has far-reaching benefits, whether that is enhancing wellbeing by bringing greenery into urban areas, replacing concrete with planting to reduce the risk of flooding, or bringing people together to improve community cohesion.
“It's a real privilege to be judging the national finalists who represent the the best of what can be achieved through community gardening. The thousands of volunteers deserve to be celebrated and have their tremendous efforts acknowledged."
The results of this year's competition will be announced at the Britain in Bloom awards ceremony in Llandudno, North Wales on 27 October.
The picture with people shows the judges and officers from the council meeting volunteer gardeners outside the Civic Centre in Uxbridge.