UC Master Gardeners, Santa Clara County, CA
University of California
UC Master Gardeners, Santa Clara County, CA

December Tips

Monthly tips are categorized by: To-Dos, What to Plant, or Pests and Diseases. Scroll through the list to see items in each category.

1. To-do

  • Clean Garden Beds -

    In preparation for winter, remove dead plants, leaves, fruit (mummies) and flowers from garden beds. Many insects and diseases overwinter in fallen debris. Do not compost any plant materials you think might be diseased.

    - December
  • Composting -

    Composting is a good way to repurpose yard and kitchen waste, and it provides a free method to feed plants and improve soil structure. If you are unsure about how to begin composting, take a look at this simple how-to compost page. You can also go to the UCCE Composting Education Program website to learn more about free two-hour classes offered throughout the county.

    As the weather warms up, compost piles dry out faster. Keep compost piles as damp as a wrung-out sponge to keep organisms alive and working on decomposing yard waste. Turning the pile to incorporate more oxygen also supports life in the compost pile.

    - January, February, March, April, May, June, July, August, September, October, November, December
  • Cut Back and Divide Perennials -

    Purple needle grassWinter is a good time to cut back perennials and bunch grasses. Perennials such as hummingbird sage and most kinds of hummingbird fuchsia can be cut back all the way to the ground. Perennials like daylilies and chrysanthemums can be divided during this time.

    Bunch grasses such as purple needle grass (official California state grass), Idaho fescue and deer grass, and some perennials like douglas iris, alum root, seaside daisy and yerba buena, can be propagated by division this time of year.

    - January, February, December
  • Dormancy and Chill Hours -

    The positive side of cold weather is that fruit trees native to colder climates, such as cherries and blueberries, may get the chill hours they need to produce good fruit. Going dormant saves energy which can then go into fruit development. No need to protect them on cold nights. Knowing the native habitat of your plants will guide you in caring for them. Mimicking the success of nature leads to greater success in your own garden.

    Learn more about chill hours on the UC Fruit & Nut Information website.

    - January, February, December
  • Frost Dates and Avoiding Frost Damage -

    The first and last frost dates for Santa Clara County are November 15 and March 15. First and last frost dates are important (but approximate) dates for gardeners to remember.

    First frost date—this is the earliest date you should expect frost to occur. If you have plants that need to be brought in for the winter, or crops you need to pick before frost, this date will be important to you. Last frost date—after this date you wouldn't expect any more frosts. It's generally used as a milestone when planting outdoors, or pruning frost sensitive plants (such as citrus where you don't want to stimulate delicate new growth until danger of frost is past).

    Note that microclimates in the county, and unusual weather conditions make these approximate dates.

    When there's a threat of frost, make sure your frost-tender plants are well-watered. Soil that is damp can hold more heat than soil that is dry. According to the UC publication on the Principles of Frost Protection, ”when the soil is wet ... more heat is stored during daylight for release during the night.”

    Frosts or a hard freeze can kill tender plants and can damage citrus, especially young plants so protection is a good idea on those cold, clear winter nights. Place stakes around tender plants and cover with clear plastic or fabric such as a sheet or old drapery. Don't let the material touch the foliage.

    Wrap larger plants with strings of Christmas tree lights (incandescent bulbs not LED bulbs) or position a 100–150 watt spotlight in the center of the tree and cover the plant with a sheet. Turn the lights on at night.

    If plants are potted, then move them to a sheltered area such as a porch, under the eaves on the south side of the house or even under a tree. Be sure to uncover them during the day. Moving them indoors to a cool room would be good if possible.

    If you have plant damage from frost, wait until spring to prune. Early pruning can lead to further dieback along stems and branches because the dead material helps protect the plant from further damage.

    - January, February, March, November, December
  • Fruit Tree Dormant Care -

    If you've had major pest infestations on your fruit trees this year, now is a good time to use dormant oil sprays. These are used to reduce overwintering populations of insects. They work by smothering soft-bodied insects and eggs when applied at the proper times in the life cycles of the pests. See our Fruit Tree and Vine Care Calendar for more information.

    - December
  • Fruit Tree Grafting -

    When pruning dormant fruit trees, you many want to save cuttings (scions) for later grafting onto other fruit trees.  Grafting is a technique that allows you to have multiple varieties of compatible fruit on one tree and is a great space saver. Fruit trees can have new varieties grafted to them when they are dormant in January and February.

    Scions are available in January at the California Rare Fruit Growers (CRFG) scion exchange. Their January event also includes training classes. Check the CRFG - Santa Clara blog for the date.

    More information on grafting can be found at:

    - Grafting and Budding on the UC Home Orchard website
    - Grafting Dormant Deciduous Fruit Scions at the Californai Rare Fruit Growers website
    Budding and Grafting Demystified from the UCCE web site

    - January, February, December
  • Holiday Ideas -

    Prune your evergreen trees and shrubs now and use the cuttings for holiday wreaths.

    Christmas cactusChristmas cactus, poinsettias, cyclamen, kalanchoe, amaryllis and more help make the house look festive at this time of year. They prefer cool temperatures, so find a place for them away from a heating vent. To avoid over watering, check the soil and give them water only when dry.

    Consider a living Christmas tree that can later be planted in your garden. A smaller tree doesn’t have room for as many ornaments, but is more likely to adapt well when planted outdoors. You can even rent a tree from Our City Forest as it's on its way to a permanent home in the community.

    You can bring some of the beauty of your garden inside to decorate your home for the holidays. Consider some of these seasonal items for tree ornaments, mantle decorations, garlands, and wreaths: laurel leaves, lemons, pine cones, nuts, pyracantha berries, eucalyptus pods, grape vines, rosemary, and rose hips.

    - December
  • Paint Trees to Prevent Sunburn -

    After deciduous trees have dropped their leaves, paint tree trunks with white latex paint diluted 1:1 with water to prevent sunburn. Preventing sunburn helps trees resist invaison by shot hole borers, which are tiny beetles that bore holes and lay eggs in the cambium layer of the tree. When the eggs hatch, young larvae feed and excavate tunnel galleries in the wood. For more information about identification and control see UC Pest Note on Shothole Borer.

    - November, December
  • Poisonous Plants List -

    As we approach the holidays, there are a lot of questions on poisonous plants. Fortunately, many of these plants have a very bitter taste that limits the amount of the plant eaten.

    Poinsettia and mistletoe should be kept away from curious children, but the list includes other flowers and plants such as azaleas, calla lily, carnation, daffodil, foxglove, hydrangeas, iris, lantana, narcissus, poppy, sweet pea and tulips.

    Different parts of the plant may be toxic. Consult the UC Poison Plant web page for detailed information.

    - November, December
  • Prune Apple Trees -

    Once your apple tree loses its leaves, it's time to think about pruning. Apple trees produce fruit terminally on spurs located on wood 2 yrs. to 8 yrs. old. Weak and unproductive branches should be thinned out to allow the sunlight into the tree for good spur development. Older spurs can be rejuvenated by cutting back, especially following a light crop year. Tree height is maintained by cutting back upper branches to shorter laterals. Excessive pruning of a bearing tree can negatively affect its vigor and fruit. Consult our Fruit Tree Pruning page for more information.

    - January, February, December
  • Prune Blackberries -

    Little or no pruning is required during the first year after planting. Blackberries should be pruned as soon as the harvest is completed. All wood that has produced the current crop should be removed. The canes should be trellised immediately after pruning. Put up only the larger canes and prune the small ones. Generally, no more than 9 canes should be put up on the trellis. A fan-like arrangement is the best way to trellis the vines. Tipping (removing the end of the canes) forces out the laterals on which fruits will be borne the following season. Consult our Berries page for more information.

    - December
  • Winter Fruit Tree Pruning -

    Pruning Tools – Keep them clean and sharp

    Remember to keep your pruners and loppers sharp. Good pruners use bypass blades rather than anvil type. Anvils have only one cutting blade and one flat blade which can result in "smashing" the plant material. Sterilize the pruners or loppers between each plant and after cutting off any diseased plant material. Use a 10% bleach solution (1 part bleach, 9 parts water) or disinfectant bathroom cleaner.

    What and When to prune

    According to UC's Backyard Orchard website, "the optimum time of year to prune fruit trees is the dormant season, December, January (best) and until the middle of February." Apricots are the exception for pruning in January; they should be pruned in the summer after harvest. If you properly prune and care for fruit trees you will get the highest yield of fruit. A good rule of thumb is to prune plum, pluot, apple, and pear trees 15-20%; and peaches 50%.

    January is also a good time to prune roses. A basic guide is to keep 3-6 strong, healthy, outside canes per plant and leave 3-5 buds on each cane. Cut on the diagonal ¼ inch above an outward-facing bud. More information is available at UC Pest Note on Roses: Cultural Practices. The San Jose Heritage Rose Garden offers free hands-on pruning classes every Saturday in January and February.

    Deciduous trees can be pruned anytime during their dormant season (in winter). Prune deciduous species, such as Western spice bush, creek dogwood, Western mock orange, red-bud, maples and deciduous oaks.

    Do not prune apricot and cherry trees in winter because they are susceptible to Eutypa dieback. The best time to prune them is late August before the rainy season starts.

    While you're outside pruning, remember to pick up any rotting fruit on the ground at the same time.

    Pruning Tips:

    Make your cuts with care in order to direct the growth for next year. To have an open tree with good air circulation, make cuts above outward-facing nodes. Choose nodes where you want new growth and make a cut about a quarter inch above, refer to the image below.

    Pruning stages
    Also refer to our page on fruit tree pruning page.

    - January, February, December
  • Winter Irrigation -

    Depending on the fall weather and rain frequency you will likely set your sprinklers to water less frequently or even turn them off for a while.  It's still important to check outdoor plants to make sure they have enough water.  While they need less water when it's cool, it's important to make sure they don't dry out.  If you have a lawn and rains haven't come, irrigate the lawn once or twice this month.

    If it has been raining, the soil may be saturated so be careful if you have to walk on it so as not to compact it.  Also if soil is waterlogged, vital space for air that is needed for plants and worms and excess water can drown beneficial soil organisms and contributed to rotting roots.

    - January, February, December

2. What to plant

  • Amaryllis or is it Hippeastrum? -

    The showy red Amaryllis (more correctly called Hippeastrum) is a great bulb for growing indoors if you can’t wait for your outdoor bulbs to bloom in spring. Choose a pot just slightly larger than the bulb. Plant it in loose potting soil with a third of the bulb sticking up above the soil surface. Keep moist, but not so wet as to rot the bulb. For repeat blooming, refer to the National Arboretum's tips on How to Make Your Amaryllis Bloom Again.

    Photos: UC Solano Master Gardeners, Growing Amaryllis or is it Hippeastrum?")

    - December
  • Bare Root Plants -

    Bare root plants are sold without any soil clinging to the roots making them easier and less expensive to transport; they'll do just fine in the garden as long as you don't let them dry out before planting. Because you can see the roots and can control how they're placed in the soil, it helps reduce the chances for root girdling problems later. Buy and plant early in the month while roots are still fresh.

    The bare roots should be soaked from an hour to overnight (large plants) in a bucket of water before planting. Trim roots of broken, dead or spongy bits and carefully pull the roots apart. Dig a hole that is fairly shallow and wide. Spread the roots out sideways and have the crown of the plant several inches above the soil level. This is necessary as the plant will settle down over time. Water in well but wait to fertilize until you see new shoots growing. Be sure to water regularly if the rains are sparse. Staking may not be necessary.

    Trees aren't the only plants that are sold bare root. You can also plant bare root asparagus, artichokes, rhubarb, berries, kiwifruit, horseradish, rhubarb, grapes, roses, strawberries, and iris in January.

    - January, February, December
  • Planting Onions -

    Red onionGrowing your own onions can be rewarding. According to our Onion Cultivation Tips, plants started from seed in September should be ready to transplant in 50-60 days, so plan for November-December for planting at home.

    The UC Davis Vegetable Research and Information Center also has information about Growing Onions.

    The image shows a 3+ pound onion harvested in June 2016 at our Berger Edibles Garden.

    - November, December
  • Poinsettias -

    Poinsettia growing outdoors in Pismo BeachPoinsettias are a tropical plant so they'll do best in a warm sunny place in your house. The soil can easily dry out and become hydrophobic, so make sure to check the soil moisture regularly. There’s really no need to fuss over them if you’re only keeping them for a few weeks of holiday color. If you want them to bloom again next year, UC has Poinsettia Care Tips that describes what to do.

    Poinsettias are beautiful at this time of year due to being forced in commercial greenhouses. If you want to save yours for next year, you will also need to take measures to force bloom in time for the holidays (it's a little tricky, but here's how if you're interested). Did you know that the red parts are actually bracts (modified leaves) and that the true flowers are the little yellow parts in the centers? You can also plant them outdoors with protection from frost. They have been seen growing as tall as eight feet in San Jose, and taller in their native Mexico.

    - December

3. Pests and Diseases

  • Cabbageworms -

    Examine your cabbage and broccoli plants for imported cabbageworms. Larvae are green and very hairy, with an almost velvet-like appearance. Handpick the worms. The adult cabbage butterfly is white with one to four black spots on the wings; they are often seen fluttering around the fields. Whitish, rocket-shaped eggs are laid singly on the undersides of leaves. The cabbageworm is active throughout the year in California. Use row covers or screening to keep the adults out of your vegetables. For more information see UC Pest Note on Imported Cabbageworm.

    - October, November, December
  • Limes with Stylar End Rot -

    Lime with stylar end rotUsually affects Persian and Bearss limes but can be on other limes and lemons. Depressed areas are firm or leathery and dry. It starts out looking like a water soaked, whitish-to-drab sunken patch at the base of the stylar tip (end) and can cover 1/4 to 1/2 of the fruit. Inside tissues at stylar end break down and become pinkish or brownish.

    You can pick before mature to cut losses. Extremely high temperatures can bring it on.

    - November, December
  • Mistletoe -

    Mistletoe is a parasitic plant that reduces the vigor of the host tree and can threaten its survival, especially if it's drought weakened.

    Cut off affected limb 18" or more below mistletoe attachment. If this isn't possible, remove mistletoe and wrap the infected area with black plastic to kill any resprouting. More information about this plant is in the UC Pest Note on Mistletoe.

    - December
  • Oak Root Fungus -

    This fungal infection, Armillaria Mellea, also known as mushroom root rot, honey fungus and shoestring fungus can cause foliage to become sparse and eventually kill the tree. The white or cream-colored fungus will grow between bark and wood of roots and up into the lower part of the tree trunk. It smells like your typical grocery store mushroom.

    In spring or fall there may be mushrooms growing around the base of the infected tree. If the disease has not progressed too far, you can cut out infected tissue and expose the base of the tree and roots to air. Reduce watering especially close to trunk. No chemical treatment is known to be effective.

    Plants Resistant or Susceptible to Armillaria mellea, The Oak Root Fungus by Robert D. Raabe contains more detailed information. The UC Master Gardener video Revealing Root Rot can help with detection of two common root pathogens and provide guidance on how to reduce the spread.

    - December
  • Peach Leaf Curl Preventive Care -

    If you had a serious problem this past year with peach leaf curl, where the leaves became red and distorted, spray the tree with a fungicide once all the leaves have fallen (late November, early December).

    Generally a single early treatment when the tree is dormant is effective, although in areas of high rainfall or during a particularly wet winter, it might be advisable to apply a second spray late in the dormant season, preferably as flower buds begin to swell but before green leaf tips are first visible.

    Use a fixed copper spray, as detailed in UC Pest Note on Peach Leaf Curl

    - November, December
  • Peppertree Psyllids -

    This pest can cause damage to California pepper trees. The damage is done when the insect is in its immature stage. They make doughnut-like pits on leaves that wrinkle or twist the leaves as well. Parasitic wasps can provide satisfactory control. See the UC Pest Note on Psyllids for more information.

    - December
  • White Mold on Lettuce and Brussels Sprouts -

    White mold is a distinctive disease that most often affects stems and foliage at the base of cole crops* and lettuce plants. Affected tissue develops a soft, watery rot and white, cottony mycelium forms on the surface. Plants may wilt if stems are girdled by the decay. As affected tissue dries up, it turns yellow to white, and hard black sclerotia form on the surface or inside the dead stems. Get more information in the UC Pest Note on White Mold.

    * Cole crops include broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, and kohlrabi

    - November, December

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