For folks who have pluots or plums, even peaches and other stone fruits that bloomed early in the season only to be damaged by late frosts, I’ve read articles about cold hardening them and offering some protection when buds start to swell earlier than usual.
The trick is to use a 2%-4% urea sprayed on the buds a few times during the warmest part of the week. Also add an ounce of glycerine per gallon of the spray solution to provide some sticker effect and for easier absorption of urea into the buds and stems. This hardens the buds against the cold and both the urea and glycerine provides some cryogenic protection.
You can use up to 10% urea solution before the buds swell, 4% urea solution if the bud is just starting to swell, 2% if it is starting to break out.
Adding pulverized aspirin (1,500 mg per gallon solution) will also help as it provides salicylic acid and allows plants quicker recovery in case you have severe cold damage. The aspirin also have antifungal properties.
I have successfully tried it on some of the Japanese Mume that I have when there’s a bud swell in December after a mild heat wave and winter has just started. I still managed to get a decent fruit sets even if there were two arctic blasts and freezing events later that winter.
I have used the 1.5% urea solution with aspirin and it also protects my avocados and citruses during freeze events with minimal damage to leaves.
I’ve also come across such research in protecting peach blooms from the forecasted freeze events. May not be total protection, but it helps a lot.
You should buy the Low Bieuret Urea. You can get it from Amazon or eBay. The cheapest one is from a local agricultural store or if you have a farmer friend, you can ask for some.
Here’s one of the peer reviewed references, sorry I don’t have paid subscription but I pasted the abstract here:
Foliar applied urea improves freezing protection to avocado and peach☆
Author panel
S.ZilkahZ.WiesmannI.KleinI.David
Abstract
The effect of foliar applied urea on freeze hardiness was evaluated under orchard and laboratory conditions. Freezing injury and senescence of avocado (Persea americana Mill.) cv. ‘Hass’ leaves in the orchard was inversely correlated to N content. Three foliar applications of 2% low-biuret urea caused a 26% nitrogen enrichment of leaves. Consequently, leaf freezing hardiness was increased and senescence retarded. Urea-treated leaves were 2.5 times more tolerant to freezing than untreated leaves, at the same level of senescence. The freeze protection effect of foliar-applied urea was confirmed using potted plants of the cultivar ‘Fuerte’ under controlled conditions. Potted avocado plants treated with 2% low-biuret urea, gradually cooled, and exposed to −2 °C for 4 h, were significantly more hardy than control plants. Foliar application of 10% low-biuret urea to peach (Prunus persica L.) cv. ‘Maravilha’ shoots before flowering, 3 days before frost occurrence, increased the freezing hardiness and survival of the reproductive organs. The protective effect of foliar-applied urea on peach flower buds in spring decreased gradually as bud development progressed.