Great Question! My Response to Reader Q’s

A friend and neighbor recently asked me for guidance on growing in our area, specifically along “West Lilac,” the road that connects our two homes. West Lilac is a microzone within Zone 10a of San Diego, characterized by beautiful hills and chaparral with views of valleys and the ocean. We’re situated about a dozen miles inland, where it’s usually warmer and sunnier during the day and cooler at night than the coast. Yet we still experience coastal fog, which hugs the banks of the rivers and streams that run from the mountains through our hills and into the Pacific Ocean.

As a gardener, I appreciate the idea of working in harmony with your exact location, so my advice below is based on my experience in Bonsall, California.

Reader Question: When should we plan to grow most crops/flowers in our garden for our area? I know about cool and warm season crops, but can’t we start growing just about anything because we hardly get frost in our area? Yes, the days are shorter, but aren’t growing seasons more likely to do with frost?

Answer: First, I’m glad you listened in seed-starting class about warm and cool seasons! And the answer is yes and no. You can grow almost anything year-round, but it would take some sort of season extender (greenhouse, row cover, shade cloth), and it’s most challenging to do for cool-season crops. It’s easier to warm up a San Diego greenhouse in January than run an air conditioning unit in your garden in August.

It’s easier to warm up a San Diego greenhouse in January than run an air conditioning unit in your garden in August.

Air temperature. Heat is an essential factor influencing crops. For example, ranunculus goes dormant when temperatures stay 80 degrees and above. I planted ranunculus bulbs in late December that should bloom in late February or early March. In our location, March high temperatures range from 67 to 69 degrees, with lows ranging from 46 to 48. That’s a good range for ranunculus. In April, highs average 69 to 72 degrees, although there are some years where we’ve had highs in the 80s. If that happened for a long stretch this year, my ranunculus would be done. If we have a “normal” spring, I’ll have blooms into early May. Next year I’ll adjust my schedule and plant ranunculus in November, so blooms start in late February. This will give me a longer growing season.

MonthAverage Highs in Bonsall, Calif.
January66-67 F
February67 F
March67-69 F
April69-72 F
May72-74 F
June74-80 F
July80-82 F
August82-83 F
September79-83 F
October74-79 F
November68-74 F
December66-68 F
These numbers come from WeatherSpark. I also checked the Century 21 real estate site, which gathers data from Camp Pendleton MCAS reporting, and the ranges were similar.

Clearly our neighbors, who are flower farmers here in Bonsall, understand when to plant ranunculus. I shot this photo in early April, when blooms peaked. I hope my ranunculus look this gorgeous come spring!

The same goes for many cool-season crops like snapdragons and sweet peas, which don’t tolerate the heat. In June, high temperatures range from 74 to 80 degrees. Sweet peas prefer to flower at 48 to 68 degrees, and their archenemy, downy mildew, thrives at 68 to 77 degrees. Guess who will win come June. The sweet peas grow weaker, and the downy mildew gets stronger. I could spray an organic fungicide, but I prefer not to use sprays and would rather grow crops within their comfort zones when immune systems are naturally vigorous.

Sweet peas prefer to flower at 48 to 68 degrees, and their archenemy, downy mildew, thrives at 68 to 77 degrees. Guess who will win come June.

Soil Temperature. By the way, vegetables have similar preferences, leading us to a discussion about soil temperatures. Oregon State University has a guide called “Let Soil Temperatures Guide You When Planting Vegetables.” They recommend 50-degree soil temperatures for cool-season crops, while warm-season crops prefer temperatures above 60. And for tomatoes, 65 to 70 degrees is ideal. You can warm up soil temperatures using cloches, mulch or row cover.

If you love a type of crop, you can sometimes find varieties suited to more extreme weather than the crop might typically prefer. I like Muir lettuce from Johnny’s Selected Seeds, for example. While lettuce is a cool-season crop, Muir is the most heat tolerant variety I’ve found, and I can often grow it in late spring and early summer in a cooler patch of the garden.

These were the last of our sweet peas in 2023, photographed the first week of June. We grew these under shade cloth to cool them off a little.

Day Length/Dark Length. There’s also the subject of day length, as you mentioned. The way an organism responds to day length is called photoperiodism. One amazing example here in San Diego and across the South is the onion. In San Diego, we grow short-day onions.

Short-day onions need 10 to 12 hours of day length to bulb, whereas long-day onions need 14 to 16 hours of day length to bulb. In the North, summer days are long, but in San Diego, the longest day we experience (June 21) is approximately 14 hours and 19 minutes. And that’s just one day. We have a handful of days that exceed 14 hours, but not enough to make a long-day onion grow a big bulb. Instead, we’d get some lovely greens on top and a teeny tiny bulb.

I hope that answers the question. As you can see, the answer is “yes and no” to growing crops year-round. You have options if you have equipment like a greenhouse or high tunnel. But if you’re gardening in your backyard, I’d stick within plants’ comfort zones and use season extenders like row cover and shade cloth to keep crops at their happiest and most productive.

For information on seeding and planting times for individual crops, I recommend the San Diego Seed Company calendar and the San Diego favorite tome, Pat Welsh’s Southern California Organic Gardening: Month by Month.

I hope this information is helpful. Stay tuned for more answers in the next post!