How recent rains will impact summer gardening in Southern California

A winter season marked by multiple back-to-back atmospheric rivers gave way to Instagram-perfect green hillsides, superblooms and refilling reservoirs. But for homeowners and gardeners, challenges still remain.

Experts say that while trees, shrubs and other plants benefited from all the rain, the cold and wet weather has also resulted in plants developing more slowly and blooming later as well as more weeds. And gardeners will still want to take steps to protect their plants from the hot and dry weather that will come this summer — and beyond.

So here are some areas to keep in mind:

1. Watch out for weeds, diseases and pests

Janet Hartin, a UC Cooperative Extension environmental horticulture advisor for San Bernardino, Los Angeles, and Riverside counties, said that when weeds produced seeds in recent seasons, drought-stricken soil layers prevented the seeds’ germination. But they were still viable when the rains came.

"Now we have this perfect storm," she said.

For keeping the weeds at bay, Hartin recommended using a mulch of some kind. She said organic mulches such as wood chips or wood shavings should be put in planter areas in 3-4 inch layers. Inorganic mulches such as decompressed gravel or pebbles can be put in 1-2 inch layers since those materials form a tighter surface that lets less light in.

Hartin said mulches have the added benefit of keeping moisture in the ground.

In addition to the weeds, there may be more invasive insect and disease pressures that gardeners have to contend with. Hartin said keeping plants healthy and well-cared for will help to stave off disease.

But disease may not even be that much of an issue, Hartin said.

In other recent years, trees and shrubs were weakened going into spring and summer because of drought conditions, but that's not the case this year.

"The plants have better protection because they’re already healthier," she said.

Gardeners who need help diagnosing a pest or disease problem have some options at their fingertips, Hartin said.

The University of California Integrated Pest Management Plant Problem Diagnostic tool can help gardeners figure out what's wrong with their edible and landscape plants. Users can search by plant name, type or symptoms.

The International Society of Arboriculture has a list of certified arborists that can provide expert help with tree problems. This may be a good option for people who have prized trees that are older or larger, Hartin said.

Lastly, the University of California Master Gardener volunteers are available to answer questions through free email and phone helplines. These services are available in all Southern California counties.

2. Don't be daunted by late blooms or stunted plants

Exequiel Ezcurra, a professor of Ecology at the Department of Botany and Plant Sciences at UC Riverside, said this spring was not only anomalous for all the rain we received but also for the cooler temperatures that persisted through late April and early May. He said evidence of the cooler weather is visible in trees that are tropical in origin: Trees such as jacarandas, tipuanas and coral trees are not forming blooms as early as they normally do. Even native plants such as the Southern California flannelbush are making blooms later than normal.

Ezcurra said that if a tree or shrub that blooms typically around this time of year hasn't yet, it likely will – just a little later than usual.

Some spring and summertime veggies may be stunted, too, according to Ezcurra.

He used the example of a garden that he maintains with his wife that has corn, beans and squash in it. He said those things, planted in late February, have not taken off like they normally do.

The corn, which would normally be ready by June, is less than two feet tall.

But Ezcurra said there's still plenty of time to grow summer veggies. If those things haven't been planted yet, they can be planted now.

For those who have planted summer vegetables already and find that they’re a little lackluster, Ezcurra's advice is to wait.

"Just water them because now that we’re getting the heat coming in they will grow and develop," he said.

3. Now is a good time to plant trees

It may not only be a good time to plant summer veggies, but also trees.

Hartin said she always recommends that people take a look at their yards and consider whether they have space for a tree because of the cooling effects trees have on urban heat islands.

They should have about 15 feet of space around where they want to plant the tree, and Hartin recommends climate-resilient species such as desert willows, mesquites and chaste trees.

Hartin said gardeners who want to plant a tree should do so in the next two to three weeks, or wait until fall when the weather has begun to cool.

4. Be aware that your plants will still be thirsty

It's already getting a lot warmer and there are some indications that it could be an average or slightly warmer than average summer, according to Ezcurra.

For trees, he said, that may not mean much yet because there's still lots of water in deeper layers of soil, but annual plants with shallow root systems will soon need supplemental water from a sprinkler system if they’re not already getting it.

As the summer progresses, shrubs and trees will start drawing from those lower soil moisture levels.

Hartin said that July will be the highest water-demanding month and that it will be important to deep water these plants. She said they should be allowed to dry between waterings to avoid disease-causing fungi.

Hartin also noted that while much of Southern California is no longer in a drought, we’re not out of the woods yet.

She noted that while rainfall was considered average for places such as Coastal Orange County, dryer than normal conditions persist in much of Riverside and San Bernardino counties and desert areas are still in drought.

"I think we need to set our expectations with an eye on, ‘Could this continue?’" Hartin said. "And I think that it's likely the drought will continue."

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