Growing Guide: What Flowers to Plant in August (USDA Zones 9–10, Bay Area)

The Bay Area in August is a tale of microclimates — foggy coast, mild bay flats, and toasty inland pockets. The good news: you can still get a burst of color from seeds this month. Below is a simple, microclimate-aware plan you can copy straight into your garden journal (or into raised beds this weekend).

What to Plant:

Coast & Fog Belt (cooler, breezy)

• Start for fall color: Calendula (from seed or cell packs).

• Try if you have a warm pocket: Zinnia, Cosmos, Marigold—choose quick, compact varieties and give them the warmest, sunniest spot you’ve got.

• Keep soil evenly moist for germination and early growth; wind + fog can dry seedlings fast.

Bay Flats & Near-Bay Neighborhoods (mild, sunny)

• Fast annuals from seed: Zinnia, Cosmos, Marigold, Sunflower (choose 55–70-day-to-bloom types where possible).

• Also start Calendula now for early fall flowers.

• Deadhead weekly to keep color rolling into October.

Inland & Hills (hot, bright)

• Heat lovers shine: Zinnia, Cosmos, Marigold, Sunflower, Gomphrena, Celosia (direct-sow or start in cells).

• Mulch right after seedlings establish to reduce afternoon wilt.

• Water deep, early morning; consider afternoon shade cloth during heat waves.

Fast Annuals to Prioritize in August

• Zinnia (short varieties) – quick bloom, great for cutting.

• Cosmos – airy, pollinator-magnet, keeps going with deadheading.

• Marigold – heat-tough, bright borders; your garden photos will slot in perfectly.

• Sunflower (dwarf/branching) – faster to bloom than giants; ideal for late summer sowing.

• Gomphrena & Celosia – love heat, long-lasting color, excellent for bouquets.

Step-By-Step Basics

1. Prep the bed/containers

   Loosen top 6–8”, mix in compost, and rake smooth. In hot zones, pre-soak the bed the day before sowing.

   
   

2. Sow or transplant

   • Direct-sow zinnia, cosmos, marigold, sunflower where they’ll grow; thin early for sturdy stems.

   • Transplant calendula starts if you want a head start near the coast.

   
   

3. Watering (August edition)

   Deep morning water; keep seed zones consistently moist until sprouted. In inland heat, consider a temporary shade cloth for afternoon scorch.

   
   

4. Feed & mulch

   Light, balanced organic feed at 2–3 weeks. Mulch after seedlings are 3–4” tall to hold moisture and moderate soil temps.

   
   

5. Pest & disease watch

   • Aphids/whiteflies: blast with water, encourage ladybugs, use insecticidal soap if needed.

   • Mildew (coast/fog): space plants, water at soil level, remove affected leaves early.

   
   

6. Bloom on repeat

   Deadhead weekly. For cut flowers, harvest in the cool morning—buds just opening last longest in the vase.

GrowBot did some digging… If your zinnias flop in a heat wave, give them a post-lunch siesta—a scrap of shade cloth from 1–4pm. It’s like sunglasses for plants, minus the tiny nose bridge.