May Tips & Events for Santa Clara County
“Flowers are restful to look at. They have neither emotions nor conflicts.”
~Sigmund Freud
Monthly Tips
Quiz: What Are These Bumps on My Tree?
On this plum tree, you can see clusters of odd-looking bumps at the tip of the branch. You are likely to find more than one cluster on your tree, showing up where the buds form. What are these and do you need to worry about them?

Photo credit: Ying Chen
Growing Great Peppers and Tomatoes
Here are some top tips for getting great results with your peppers and tomatoes.
  • Plant them in full sun, which means at least 8 hours. It takes a lot of energy to produce large fruits like peppers and tomatoes. How do plants get that energy? Via photosynthesis using sunshine. So give them your sunniest spot.
  • Wait until the soil and air temperatures are warm before planting. May 1 is a good rule of thumb. Cold conditions can damage plants or make them prone to diseases.
  • Provide supports. Indeterminate tomatoes can reach 6+ feet and some peppers can get heavy enough to break a branch.
See our video about growing tomatoes for more info.
An assortment of colorful red and yellow tomatoes and peppers
Photo credit: Pixabay
Fire blight
Fire Blight
In late spring and early summer, when the weather is sufficiently warm and moist, you may start seeing some black or brown leaves, fruits, or branches on your tree, accompanied by a light tan ooze. This might be fire blight. It is a bacterial disease that apples and pears are most susceptible to. It is spread by insects, rain, or pruning. The bacteria enter through the blossoms and travel down the tree. If left unchecked, fire blight can enter the trunk and kill the entire tree. Prune infected branches back to healthy wood, at least eight inches below visible damage. If the inside of the branch is discolored, you need to cut back still further. Clean pruning tools between cuts so as not to spread the infection. Planting varieties that are less prone to fire blight is helpful. Always promptly clean up fallen fruit and leaves.

Photo: Tree shoots killed by fire blight, by Jack Kelly Clark, UC
Oh No! Ants!
Argentine ants on avocado tree
Have you seen lines of ants scurrying along tree trunks, plants, edging, or even in your house? They’re probably Argentine ants, the most common species in California. These pests are attracted to the sweet honeydew excrement that is produced by aphids, mealybugs, and soft scale insects. They love this honeydew so much that they actually protect scales and aphids, increasing their population. What to do? Reduce their food source by managing honeydew-producing insects. Band tree trunks with sticky substances like Tanglefoot. Buy ant bait that comes pre-packaged as ant stakes or in small containers. Ants bring the bait back to their nests and feed it to others who die. Good luck!

Photo: Argentine ants on avocado tree, by Ying Chen
Will Vinegar Kill My Weeds?
Little Mallow weed seedling
Vinegar can work as a weed killer, but its effectiveness is limited. Household white vinegar (5% acetic acid) is too weak to kill anything other than tiny seedlings. Stronger horticultural vinegars with 20% acetic acid provide better weed control but typically only for annual, not perennial, weeds. And the higher concentration can be dangerous—including skin burns and eye injury—if not handled carefully. For comprehensive guidance on weed management in landscapes, refer to the University of California’s Weed Management in Landscapes recommendations.

Photo: Little mallow weed, by Jack Kelly Clark, UC
Arborist wood chips mulch
Photo: Arborist wood chip mulch, University of California Agriculture and Natural Resources
Free Wood Chips from Arborists Make Good Mulch
You probably already know the benefits of mulch—it suppresses weeds, regulates soil temperature, and if organic, decomposes over time to enrich soil. Did you also know you can find free wood chip mulch from local arborists by searching online? (Try searching for “free wood chips near me.”) These wood chips are among the best mulch choices for trees and shrubs. Unlike the uniform texture of some other mulches, arborist wood chips include bark, wood, and leaves that resist compaction and host a variety of microbes. There are other good mulch choices, depending on where you use it. And spring is the ideal time to renew your mulch as it has probably decomposed with winter rains. Add new mulch to reach a depth of 3–4 inches.
Quiz Answer: Plum Bud Galls
These are galls formed by a microscopic pest called plum bud gall mite. You won’t be able to see them without magnification, but their feeding causes the galls to form on the tree. These galls are made up of different chambers and these chambers can be filled with a large population of mites. The mites are primarily dispersed by wind but can be spread by insects, birds, and infected budwood. Heavy infestation can weaken trees, reduce yields, and in some cases, cause the death of the tree. If you suspect that your tree is affected by these mites, you should avoid moving budwood off your property to keep from spreading the pest. Because they are a new pest, options for managing them are still being developed. However, targeted pesticide application can be very effective when applied in the spring season during bud break when mites emerge from infected galls.
Magnified image of plum bud gall mite
Photo: Eriophyid mite appearance under magnification, by Eric Erbe, digital colorization by Chris Pooley (USDA, ARS, EMU).
Upcoming Events
No plant clinic in May.

PADG Waterwise Gardens: More Beauty, Less Water, Saturday, May 4, 10–11 am, Palo Alto Demonstration Garden, 851 Center Drive, Palo Alto

Growing Summer Flowers, Herbs and Vegetables in Containers, Wednesday, May 8, 6:30–7:30 pm, Morgan Hill Public Library, 660 W. Main Avenue, Morgan Hill

Growing Culinary Herbs, Saturday, May 11, 10 am–noon, McClellan Ranch Preserve, Community Garden, 22221 McClellan Road, Cupertino

MCP Seed Swap: Soil, Sow & Sip, Saturday, May 11, 11 am–1 pm, Master Gardener Teaching Pavilion at Martial Cottle Park, 5283 Snell Avenue, San Jose

Tomato Talk, Saturday, May 11, 11 am–noon, Santa Clara Central Park Library, 2635 Homestead Road, Santa Clara

Managing Pests and Welcoming Beneficial Insects to Your Garden, Tuesday, May 14, 6:30–7:30 pm, First Floor, Mountain View Public Library, 585 Franklin Street, Mountain View

Wonderful Pollinators!, Wednesday, May 15, 7–8:30 pm, Los Altos Library, 13 S. San Antonio Road, Los Altos

Moles- Voles- and Gophers!- Online, Thursday, May 16, noon–1 pm, Online

MCP Succulent Open House & Sale, Saturday, May 18, 9 am–noon, Martial Cottle Park, 5283 Snell Avenue, San Jose

SCTDG Using Succulents in Container Gardening, Saturday, May 18, 9:30–11 am, South County Teaching and Demonstration Garden, 80 Highland Avenue (Corner of Monterey Road & Highland Avenue), San Martin

Year Round Flowers for Pollinators in Your Garden, Saturday, May 18, 10:30–11:45 am, Willow Glen Public Library, 1157 Minnesota Avenue, San Jose

Container Gardening for All Reasons, Thursday, May 23, 4:30–5:30 pm, Rinconada Library, 1213 Newell Road, Palo Alto

Growing Peppers !, Saturday, May 25, 10:30 am–noon, Garden to Table Silicon Valley, 200 W. Taylor Street, San Jose
Check our calendar for the latest schedule of events. Videos of many past presentations are also available.
About Us
University of California Master Gardener volunteers promote sustainable gardening practices and provide research-based horticultural information to home gardeners. Visit our website for more information including:

Have a gardening question? Ask us. We’re here to help!
Connect With Us