Holly berries are fairly persistent in clinging to Ilex tree and shrub branches, until winter weather or foraging birds finally displace them. Hollies are also quite susceptible to drought conditions, and though established plants survive drought stress, their way of coping can result in thinning and dropping of foliage and berries. Limiting premature berry drop centers on perfecting the cultural conditions for the plant so it is not under stress and driven to ration its limited resources. Moisture and nutrients are the keys and must be tended to year-round, particularly as the berries are forming. If berries are lost from the plant when young, they will not be replaced until the following year.
Water your holly shrubs deeply and regularly to compensate for a lack of rain as needed. Holly prefers moist to quite wet soil so any drying of the soil below the top inch or two could be sensed as the start of a drought. Keep the soil evenly moist at all times and use your hands to determine when watering is required. When soil feels very moist to wet at 10 inches down, watering is not required. When dry at an inch or two down, water immediately.
Mulch around the base of your holly plants to hold moisture in the soil and keep weeds away, both of which reduce stress on the plant and help preserve the largest amount of foliage and berries on the branches. Lay down 2 to 3 inches of shredded bark, cocoa bean hulls or leaf mold once a year, keeping the mulch at least 6 inches out from the main trunks.
Fertilize your holly plants once or twice each year with an organic fertilizer designed for hollies or other acid-loving plants.. Look for a product with a guaranteed analysis of 4-3-4 or a similar ratio that also contains micronutrients. Apply according to the label dosing directions and water in well. Ample desirable nutrients in the soil along with plentiful moisture will reduce any stress on the holly plant, allowing it to hang onto the largest harvest of berries possible with no need to drop them to compensate for that stress.